Vacuum degassing apparatus and vacuum degassing method for molten glass

ABSTRACT

A vacuum degassing apparatus for molten glass is comprised of an uprising pipe, a vacuum degassing vessel, a downfalling pipe, an upstream side pit that supplies molten glass to the uprising pipe, and a downstream side pit that receives molten glass from the downfalling pipe. The vacuum degassing apparatus for molten glass is further comprised of a separating mechanism that separates a part of molten glass moving from the downfalling pipe to the downstream side pit, and a returning pipe that returns separated molten glass to the upstream side pit.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of and claims the benefits ofpriority to U.S. Ser. No. 12/853,408, filed Aug. 10, 2010, which is acontinuation of PCT/JP09/052,810, filed Feb. 18, 2009, and claims thebenefit of priority to Japanese Patent Application No. 2008-046247,filed Feb. 27, 2008. The entire contents of all of the aboveapplications are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a vacuum degassing apparatus for moltenglass and a vacuum degassing method for molten glass.

BACKGROUND ART

Heretofore, in order to improve the quality of formed glass products, arefining step is carried out to remove bubbles generated in molten glassproduced by melting a raw material in a melting furnace, before shapingthe molten glass in a forming apparatus.

In this refining step, there has been known a method of adding e.g.sodium sulfate (Na₂SO₄) as a refining agent into a raw material inadvance, melting the raw material to produce molten glass, retaining andmaintaining the molten glass at a predetermined temperature, and therebymaking bubbles in the molten glass grow and move up by the refiningagent, to remove these bubbles.

Further, there has been known a vacuum degassing method of introducingmolten glass in a vacuum atmosphere, growing bubbles present in a flowof molten glass continuously flowing in the vacuum atmosphere, therebymaking the bubbles move up and break to remove the bubbles, followed byexhausting the molten glass from the vacuum atmosphere.

In the vacuum degassing method, a molten glass flow is formed and themolten glass moves in a vacuum atmosphere, specifically, in a vacuumdegassing vessel inside of which is maintained to a predetermined vacuumdegree. When the molten glass moves in the vacuum degassing vessel,bubbles contained in the molten glass are grown remarkably relatively ina short time, so that the grown bubbles move up in the molten glass bytheir buoyance forces and destroyed at a surface of the molten glass,thereby to remove the bubbles from the molten glass surface efficiently.

In such a vacuum degassing apparatus, the material constituting aconduit for molten glass such as a vacuum degassing vessel, an uprisingpipe or a downfalling pipe, that constitutes a flow path for moltenglass, is required to be excellent in heat resistance and corrosionresistance against molten glass. As a material satisfying thisrequirement, platinum or a platinum alloy such as a platinum-rhodiumalloy, or a refractory bricks such as electrocast bricks, are employed.

These materials are materials excellent in heat resistance and corrosionresistance against molten glass, but in each of the cases where theconduit for molten glass is made of refractory bricks, platinum or aplatinum alloy, bubbles may be generated on an interface between aconduit wall face and molten glass. When such generation of bubbles onthe interface between the conduit wall face and the molten glass occursin a vacuum degassing vessel (particularly on the downstream side of thevacuum degassing vessel) or in a downfalling pipe, it is difficult toremove bubbles from the molten glass, which causes defects in productglasses.

As described above, in order to remove bubbles in molten glassefficiently and securely, a process of growing bubbles in a molten glassand making the bubbles move up and break on a surface of the moltenglass, is necessary. In order to conduct such a process securely andefficiently, it is necessary to maintain the degree of vacuum in thevacuum degassing vessel within a proper range.

In the vacuum degassing method for molten glass described in PatentDocument 1, in order to always maintain the degree of vacuum in a vacuumdegassing vessel within a proper range, it is proposed to compensate thedegree of vacuum in the vacuum degassing vessel in accordance withchange of a barometric pressure. However, when the degree of vacuum inthe vacuum degassing vessel is compensated, the level of molten glass inthe vacuum degassing vessel changes to affect the effect of vacuumdegassing. Accordingly, in the vacuum degassing method for molten glassdescribed in Patent Document 1, when the degree of vacuum in the vacuumdegassing vessel is compensated, it is proposed to move up and down theposition of the vacuum degassing vessel to maintain the level of moltenglass in the vacuum degassing vessel to be constant.

In the method described in Patent Document 1, while the level of moltenglass in the vacuum degassing vessel is maintained to be constant, thedegree of vacuum in the vacuum degassing vessel is always maintainedwithin a proper range, whereby it is possible to maintain the effect ofvacuum degassing always in an optimum condition.

However, it is not possible to move up and down a vacuum degassingvessel in every vacuum degassing apparatus. For example, in a case ofemploying a large-sized vacuum degassing vessel to increase thedegassing capacity of molten glass, it is extremely difficult to move upand down such a vacuum degassing vessel in accordance with compensationof the degree of vacuum in the vacuum degassing vessel, and such amethod is not practical.

Further, in a case of a vacuum degassing apparatus having a structurethat an uprising pipe and a downfalling pipe are fixed to an upstreamside pit and a downstream side pit, respectively, such as the vacuumdegassing apparatus described in Patent Document 2, it is not possibleto move a vacuum degassing vessel up and down.

In such cases of a vacuum degassing apparatus having a vacuum degassingvessel that cannot be moved up and down, when the degree of vacuum inthe vacuum degassing vessel is compensated in accordance with change ofbarometric pressure, the level of molten glass in the vacuum degassingvessel changes to affect the effect of vacuum degassing. Particularly,when the level of molten glass in the vacuum degassing vessel rises, thedistance from the bottom of the vacuum degassing vessel to the level ofmolten glass increases, which prevents bubbles present in the vicinityof the bottom of the vacuum degassing vessel from moving up, anddecreases the effect of vacuum degassing. When a vacuum degassing vesselcannot be moved up and down, it is difficult to adjust the pressure atthe bottom since it is determined by the depth of the bottom from thelevel of molten glass in the vacuum degassing vessel.

-   Patent Document 1: JP-A-2006-306662-   Patent Document 2: JP-A-2000-7344

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In order to solve the above problems of conventional techniques, it isan object of the present invention to provide a vacuum degassingapparatus for molten glass and a vacuum degassing method for moltenglass, which can suppress generation of bubbles on an interface betweenmolten glass and a wall face of a conduit for molten glass such as avacuum degassing vessel, an uprising pipe or a downfalling pipeconstituting the vacuum degassing apparatus, which can suppress aninfluence of lowering of vacuum degassing effect due to rise of thelevel of molten glass in the vacuum degassing vessel, and which canthereby exhibit the effect of vacuum degassing stably.

Means for Solving the Problems

In order to achieve the above object, the present invention provides avacuum degassing apparatus for molten glass, comprising an uprisingpipe, a vacuum degassing vessel, a downfalling pipe, an upstream sidepit for supplying molten glass to the uprising pipe, and a downstreamside pit for receiving molten glass from the downfalling pipe, whereinthe vacuum degassing apparatus for molten glass further comprises aseparating mechanism for separating a part of molten glass moving fromthe downfalling pipe to the downstream side pit, and a returning pipefor returning molten glass separated by the separating mechanism to theupstream side pit (hereinafter referred to as “vacuum degassingapparatus of the present invention”).

A first embodiment of the vacuum degassing apparatus of the presentinvention is such that the downstream side pit has a side portion havingan opening forming an end of the returning pipe,

a conduit structure for molten glass comprising a hollow pipe made ofplatinum or a platinum alloy, a part of which functions as theseparating mechanism, is connected to a downstream end of thedownfalling pipe,

the conduit structure has a double pipe structure comprising an innerpipe and an outer pipe, formed at least in the downstream end sideportion of the conduit structure,

the inner pipe has an upstream end and a downstream end that are openends,

the outer pipe has an upstream end, that is an open end, and adownstream end, that is a closed end, the inner pipe perforating througha part of the closed end, and

an opening is provided in a downstream end side of the outer pipe at aposition facing to the opening provided in the side portion of thedownstream side pit.

A second embodiment of the vacuum degassing apparatus of the presentinvention is such that an opening forming an end of the returning pipeis provided in a side portion of the downstream side pit,

the downfalling pipe has a conduit structure for molten glass comprisinga hollow pipe made of platinum or a platinum alloy and having a partfunctioning as a separating mechanism,

the conduit structure has a double pipe structure comprising an innerpipe and an outer pipe, formed at least in the downstream end side ofthe conduit structure,

the inner pipe has an upstream end and a downstream end that are openends,

the outer pipe has an upstream end, that is an open end, and adownstream end, that is a closed end, the inner pipe perforating througha part of the closed end, and

an opening is provided in a downstream end side of the outer pipe at aposition facing to the opening provided in the side portion of thedownstream side pit.

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the inner pipe protrudesfrom the closed end of the outer pipe at the downstream end side of theconduit structure.

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the distance L_(in) (mm)from the upstream end of the inner pipe to the upstream end side of theopening provided in the downstream end side of the outer pipe, and theinner diameter D_(in) (mm) of the inner pipe, satisfy the relationrepresented by the following formula:

L _(in) >D _(in)/2

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the difference D_(out-in)(mm) between the inner diameter of the outer pipe and the outer diameterof the inner pipe, and the inner diameter D_(in) (mm) of the inner pipe,satisfy the relation represented by the following formula:

D _(out-in)/2≧0.02×D _(in)

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the distance L_(in) (mm)from the upstream end of the inner pipe to the upstream side end of theopening provided in the downstream end side of the outer pipe, and thedifference D_(out-in) (mm) between the inner diameter of the outer pipeand the outer diameter of the inner pipe, satisfy the relationrepresented by the following formula:

L _(in)≧(D _(out-in)/2)×3

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the cross-sectional areadifference S_(out-in) (mm²) obtained by subtracting the cross-sectionalarea of the flow path in the inner pipe from the cross-sectional area ofthe flow path in the outer pipe, and the cross-sectional area S_(in)(mm²) of the flow path in the inner pipe, satisfy the relationrepresented by the following formula:

S _(out-in) ≦S _(in)

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the area S (mm²) of theopening provided in the downstream end side of the outer pipe, and theinner diameter D_(out) (mm) of the outer pipe, satisfy the relationrepresented by the following formula:

S≧9×D _(out)

In the first and second embodiments of the vacuum degassing apparatus ofthe present invention, it is preferred that the upstream side end of theopening provided in the downstream end side of the outer pipe is locatedat a position lower by 0 to 500 mm than the upstream side end of theopening provided in the side portion of the downstream side pit.

A third embodiment of the vacuum degassing apparatus of the presentinvention is such that the downfalling pipe and the downstream side pitare connected so as to communicate with each other, the downstream sidepit has a double pipe structure comprising a pit main body being anouter pipe and an inner pipe located inside the pit main body andextending in the downstream direction, an opening forming an end of thereturning pipe is provided in the pit main body, and the double pipestructure functions as the separating mechanism.

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is preferred that the inner diameter D₁ (mm) of thedownfalling pipe and the outer diameter D₂ (mm) of the inner pipesatisfy the relation represented by the following formula:

D ₁ >D ₂

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is preferred that the difference ΔD (mm) between the innerdiameter of the downfalling pipe and the outer diameter of the innerpipe, and the inner diameter D₃ (mm) of the inner pipe, satisfy therelation represented by the following formula:

ΔD≧0.04×D ₃

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is preferred that the cross-sectional area difference ΔS(mm²) obtained by subtracting the cross-sectional area of the flow pathin the inner pipe from the cross-sectional area of the flow path in thedownfalling pipe, and the cross-sectional area S₁ (mm²) of the flow pathin the inner pipe, satisfy the relation represented by the followingformula:

ΔS≦S ₁

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is preferred that the downfalling pipe and the inner pipepartially overlap each other, and the length L (mm) of a portion wherethey overlap, and the outer diameter D₂ (mm) of the inner pipe, satisfythe relation represented by the following formula:

L≦5×D ₂

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is preferred that the distance d (mm) between thedownstream end of the downfalling pipe and the upstream end of the innerpipe, and the outer diameter D₂ (mm) of the inner pipe, satisfy therelation represented by the following formula:

0.5×D ₂ ≦d≦5×D ₂

A fourth embodiment of the vacuum degassing apparatus of the presentinvention is such that the opening of the returning pipe opening in thedownstream side pit satisfies the following conditions (1) and (2), andthe opening functions as the separating mechanism:

(1) the opening crosses a part of an imaginary area obtained byimaginarily extending the downfalling pipe in the downstream direction;and

(2) the opening does not cross an imaginary line obtained by imaginarilyextending the central axis of the downfalling pipe in the downstreamdirection.

In the fourth embodiment of the vacuum degassing apparatus of thepresent invention, it is preferred that the minimum distance d_(min)(mm) between the returning pipe and the imaginary line, and the radiusD_(down) (mm) of the downfalling pipe, satisfy the relation representedby the following formula:

0<d _(min) <D _(down)

In the fourth embodiment of the vacuum degassing apparatus of thepresent invention, it is preferred that an angle α (degree) between theopening and the imaginary line satisfy the relation represented by thefollowing formula:

10≦α≦80

In the fourth embodiment of the vacuum degassing apparatus of thepresent invention, it is preferred that the height of the bottom face ofthe downstream side pit is different from the height of the bottom faceof the returning pipe in the vicinity of the opening.

In the fourth embodiment of the vacuum degassing apparatus of thepresent invention, it is preferred that the bottom face of thedownstream side pit and the bottom face of the returning pipe, that havedifferent heights from each other, are connected via a slope structurehaving an angle of from 5 to 60°.

In the fourth embodiment of the vacuum degassing apparatus of thepresent invention, it is preferred that the area of the openingapproximately equals to the cross-sectional area of the returning pipe.

It is preferred that the vacuum degassing apparatus for molten glass ofthe present invention further comprises a pumping means for controllingthe flow of molten glass in the returning pipe.

It is preferred that the vacuum degassing apparatus for molten glass ofthe present invention further comprises a means for heating molten glasspassing through the returning pipe.

It is preferred that the vacuum degassing apparatus of the presentinvention further comprises a means for stirring molten glass passingthrough the returning pipe.

Further, the present invention provides a vacuum degassing method formolten glass, which is a method for vacuum-degassing molten glass bymaking the molten glass pass through a vacuum degassing vessel inside ofwhich is maintained in a vacuum state, which comprises separating a partof molten glass flown out from the vacuum degassing vessel and returningthe separated molten glass again to the vacuum degassing vessel(hereinafter referred to as “vacuum degassing method of the presentinvention”).

In the vacuum degassing method of the present invention, it is preferredthat the amount of the separated molten glass is at least 0.1% and atmost 10% of the amount of molten glass passing through the vacuumdegassing vessel.

In the vacuum degassing method of the present invention, it is preferredthat the amount of the separated molten glass is at least 1% and at most5% of the amount of molten glass passing through the vacuum degassingvessel.

In the vacuum degassing method of the present invention, it is preferredthat the ratio of the amount of the separated molten glass to the amountof molten glass passing through the vacuum degassing vessel, is changedwhile molten glass is passing through the vacuum degassing vessel.

In the vacuum degassing method of the present invention, it is preferredthat the separated molten glass is heated before it is returned to thevacuum degassing vessel.

In the vacuum degassing method of the present invention, it is preferredthat the separated molten glass is stirred before it is returned to thevacuum degassing vessel.

Effects of the Invention

In the vacuum degassing method of the present invention, a part ofmolten glass flowing out from a vacuum degassing vessel, that isspecifically a boundary laminar flow containing many bubbles caused bygeneration of bubbles on the interface between molten glass and a wallface of a conduit for molten glass or caused by lowering of vacuumdegassing effect due to rise of level of molten glass in the vacuumdegassing vessel, is split from a main flow of the molten glass andreturned to the vacuum degassing vessel to be subjected to vacuumdegassing treatment again. By this method, it is possible to suppressgeneration of bubbles on the interface between the molten glass and thewall face of the conduit for molten glass, or the influence of loweringof vacuum degassing effect due to rise of level of molten glass in thevacuum degassing vessel, to thereby stabilize the effect of vacuumdegassing. By this method, it is possible to produce glass products ofhigh quality having few defects.

Moreover, the molten glass separated from the main flow and returned tothe vacuum degassing vessel constitutes a lower layer and molten glassnewly supplied from a melting vessel constitutes an upper layer, wherebytwo layers are considered to be formed in the vacuum degassing vessel.Formation of such a two-layer flow reduces virtual depth of the moltenglass newly supplied from the melting vessel in the vacuum degassingvessel. It is expected that this improves the effect of vacuumdegassing.

Further, heretofore, the above molten glass containing bubbles has beendiscarded after the vacuum degassing. However, in the vacuum degassingmethod of the present invention, since the vacuum degassing is carriedout again as described above, the amount of molten glass to be discardedreduces, and the yield increases.

The vacuum degassing apparatus of the present invention has a separatingmechanism for separating a part of molten glass moving from thedownfalling pipe to the downstream side pit, and a returning pipe forreturning the molten glass separated by the separating mechanism to theupstream side pit, so as to efficiently separate a boundary laminar flowcontaining many bubbles from molten glass flown out from the vacuumdegassing vessel. Accordingly, the vacuum degassing apparatus issuitable for carrying out the vacuum degassing method of the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a first embodiment of the vacuumdegassing apparatus of the present invention.

FIG. 2 is a partial enlarged view showing a lower end (downstream end)side of an extension pipe 8 and its vicinity of the vacuum degassingapparatus 1 shown in FIG. 1.

FIG. 3 is a view similar to FIG. 2, which further shows dimensions ofspecific portions of the extension pipe 8.

FIG. 4 is a cross-sectional view showing another embodiment of theextension pipe, wherein the shape of the closed end of the outer pipe isdifferent from that of the extension pipe 8 of FIG. 2.

FIG. 5 is a cross-sectional view showing another embodiment of theextension pipe, wherein the shape of inner pipe is different from thoseof the extension pipes 8, 8′ shown in FIGS. 2 to 4.

FIG. 6 is a cross-sectional view showing another embodiment of theextension pipe.

FIG. 7 is a cross-sectional view showing a third embodiment of thevacuum degassing apparatus of the present invention.

FIG. 8 is a partial enlarged view showing the downstream side pit 15 andits vicinity of the vacuum degassing apparatus 1′ shown in FIG. 7.

FIG. 9 is a view similar to FIG. 8, which further shows symbols showingdimensions of specific portions in the Figure.

FIG. 10 is an enlarged cross-sectional view showing a downstream sidepit and its vicinity of another example of the third embodiment of thevacuum degassing apparatus of the present invention, wherein therelation between the extension pipe and the inner pipe is different fromthat in the construction shown in FIG. 9.

FIG. 11 is a view similar to FIG. 10, wherein the shape of the upper end(upstream end) of the inner pipe is different from that of FIG. 10.

FIG. 12 is a cross-sectional view showing a fourth embodiment of thevacuum degassing apparatus of the present invention.

FIG. 13 is a partial enlarged view showing a downstream side pit and itsvicinity of the vacuum degassing apparatus 1″ shown in FIG. 12.

FIG. 14 is a partial enlarged view showing a downstream side pit and itsvicinity of another example of the fourth embodiment of the vacuumdegassing apparatus of the present invention.

EXPLANATION OF NUMERALS

-   -   1, 1′, 1″: Vacuum degassing apparatus    -   2: Vacuum housing    -   3: Vacuum degassing vessel    -   4, 4′: Uprising pipe    -   5, 5′: Downfalling pipe    -   6: Thermal insulator    -   7: Extension pipe (uprising pipe side)    -   8, 8′, 14, 14′: Extension pipe (downfalling pipe side)    -   81, 81′, 81″, 81′″: Inner pipe    -   82, 82′, 82″, 82′″: Outer pipe    -   83, 83′, 83″, 83′″: Opening    -   9, 19: Upstream side pit    -   10, 15, 15′, 20: Downstream side pit    -   11: Returning pipe    -   12: Pumping means    -   13: Stirring means    -   18: Diameter-expanding portion    -   22: Opening    -   23: Imaginary region    -   24: Imaginary line    -   100: Melting vessel

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the present invention will be described with reference to drawings.FIG. 1 is a cross-sectional view of a first embodiment of the vacuumdegassing apparatus of the present invention. The vacuum degassingapparatus shown in FIG. 1 is employed for a process of vacuum-degassinga molten glass G in a melting vessel 100 and continuously supplying themolten glass G to a subsequent treatment vessel (not shown).

The vacuum degassing apparatus 1 has a vacuum housing made of a metalsuch as a stainless steel, inside of which is maintained in a vacuumstate at a time of operation. Inside the vacuum housing 2, a vacuumdegassing vessel 3 is contained and disposed so that its longitudinalaxis extends in a horizontal direction. To an under surface of an end ofthe vacuum degassing vessel 3, an uprising pipe 4 extending in thevertical direction is attached, and to an under surface of the otherend, a downfalling pipe 5 is attached.

In the vacuum housing 2, a thermal insulator 6 is provided around thevacuum degassing vessel 3, the uprising pipe 4 and the downfalling pipe5.

In the vacuum degassing apparatus 1, each of the vacuum degassing vessel3, the uprising pipe 4 and the downfalling pipe 5 is a hollow pipe madeof refractory bricks such as electrocast bricks, platinum or a platinumalloy.

When the vacuum degassing vessel 3 is a hollow pipe made of refractorybricks, the vacuum degassing vessel 3 is preferably a hollow pipe madeof refractory bricks, that has a rectangular external cross section, andthe inner shape forming a flow path of molten glass preferably has arectangular cross section.

When the uprising pipe 4 and the downfalling pipe 5 are each a hollowpipe made of refractory bricks, the uprising pipe 4 and the downfallingpipe 5 are preferably each a hollow pipe made of refractory bricks, thathas a circular cross section or a polygonal cross section includingrectangle as an external shape, and the internal shape forming a flowpath of molten glass preferably has a circular cross section.

On the other hand, when the vacuum degassing vessel 3 is a hollow pipemade of platinum or a platinum alloy, the internal cross-sectional shapeforming a flow path of molten glass in the vacuum degassing vessel 3 ispreferably a circular or elliptical shape.

When the uprising pipe and the downfalling pipe are each a hollow pipemade of platinum or a platinum alloy, the internal cross-sectional shapeforming a flow path of molten glass in each of the uprising pipe 4 andthe downfalling pipe 5 is preferably a circular or elliptical shape.

Dimensions of constituents of the vacuum degassing apparatus can beappropriately selected depending on the vacuum degassing apparatus to beused. In a case of a vacuum degassing vessel 3 shown in FIG. 1, thedimensions are, for example, as follows.

The length in the horizontal direction is from 1 to 30 m, preferablyfrom 1 to 25 m, more preferably from 1 to 20 m. The width of theinternal cross-sectional shape is from 0.2 to 10 m, preferably from 0.2to 7 m, more preferably from 0.2 to 5 m.

Further, a specific example of the dimensions of the uprising pipe 4 andthe downfalling pipe 5 are as follows.

The length is from 0.2 to 6 m, preferably from 0.4 to 4 m.

The width of the internal cross-sectional shape is from 0.05 to 0.8 m,preferably from 0.1 to 6 m.

To the lower ends (downstream ends) of the uprising pipe 4 and thedownfalling pipe 5, respective extension pipes 7 and 8 are attached. Theextension pipes 7 and 8 are each a hollow cylindrical pipe made ofplatinum or a platinum alloy.

The uprising pipe 4 communicates with the vacuum degassing vessel 3, andintroduces molten glass G from a melting vessel 100 to the vacuumdegassing vessel 3. For this reason, a lower end (downstream end) of theextension pipe 7 attached to the uprising pipe 4 is fit into an openingend of the upstream side pit 9, and is immersed in the molten glass G inthe upstream side pit 9.

The downfalling pipe 5 communicates with the vacuum degassing vessel 3,and discharges vacuum-degassed molten glass G to a next treatment vessel(not shown). For this reason, a lower end (downstream end) of theextension pipe 8 attached to the downfalling pipe 5 is fit into an openend of a downstream side pit 10, and immersed in molten glass G in thedownstream side pit 10.

The upstream side pit 9 and the downstream side pit 10 are made ofrefractory bricks, platinum or a platinum alloy. When the upstream sidepit 9 and the downstream side pit 10 are made of refractory bricks,their cross-sectional shapes are preferably each a polygonal shape suchas a rectangular shape, or a circular or elliptical shape for the reasonof easiness of fabrication and prevention of corrosion of refractorybricks, etc. On the other hand, when the upstream side pit 9 and thedownstream side pit 10 are made of platinum or a platinum alloy, theircross-sectional shapes are preferably each a circular or ellipticalshape for the reason of easiness of fabrication and hardness ofdeformation, etc.

As described in detail later, the extension pipe 8 forming a conduitstructure for molten glass, has a lower end (downstream end) side havinga double pipe structure which functions as a separating mechanism forseparating a part of molten glass G moving from the downfalling pipe 5to the downstream side pit 10. More specifically, a part of the moltenglass containing many bubbles caused by generation of bubbles on theinterface between molten glass and a wall face of a conduit for moltenglass or caused by lowering of vacuum degassing effect due to rise ofthe level of molten glass in the vacuum degassing vessel, is split fromthe main flow of the molten glass G moving from the downfalling pipe 5to the downstream side pit 10. A returning pipe for returning the moltenglass separated by the separating mechanism to the upstream side pit 9,connects the downstream side pit 10 and the upstream side pit 9.

The returning pipe 11 is provided with a pumping means 12 forcontrolling the molten glass flow in the returning pipe 11 and astirring means 13 for stirring molten glass passing through thereturning pipe 11. However, the pumping means 12 and the stirring means13 are not essential constituents in the first embodiment of the vacuumdegassing apparatus of the present invention, and the vacuum degassingapparatus 1 is not necessarily have these means so long as the apparatuscan exhibit intended functions without these means.

FIG. 2 is a partial enlarged view showing a lower end (downstream end)side of the extension pipe 8 and its vicinity of the vacuum degassingapparatus shown in FIG. 1.

The extension pipe 8 shown in FIG. 2 has a lower end (downstream end)side having a double pipe structure comprising an inner pipe 81 and anouter pipe 82. The inner pipe 81 and an outer pipe 82 are each a hollowcylindrical pipe made of platinum or a platinum alloy. Here, theplatinum alloy may, for example, be a platinum-gold alloy or aplatinum-rhodium alloy. “Platinum or a platinum alloy” includes areinforced platinum formed by dispersing a metal oxide in platinum or ina platinum alloy. The metal oxide to be dispersed may, for example, be ametal oxide of Group 3, 4 or 13 in the long periodic table such asAl₂O₃, ZrO₂ or Y₂O₃.

In the extension pipe 8 shown in FIG. 2, the inner pipe has an upper end(upstream end) and a lower end (downstream end) that are open ends.

The outer pipe 82 has an upper end (upstream end) that is an open end,and a lower end (downstream end) that is a closed end. The inner pipe 81perforates through a part of the lower end (downstream end) of the outerpipe 82 that is a closed end, and the lower end (downstream end) of theinner pipe 81 protrudes from the lower end (downstream end) (closed end)of the outer pipe 82. Here, in the extension pipe 8 shown in FIG. 2, thelower end (downstream end) of the inner pipe 81 protrudes from the lowerend (downstream end) (closed end) of the outer pipe 82, but the lowerend (downstream end) of the inner pipe is not necessarily protrude fromthe lower end (downstream end) (closed end) of the outer pipe. In thiscase, the height of the lower end (downstream end) of the inner pipe isequal to the height of the lower end (downstream end) of the outer pipe.The phrase “the inner pipe 81 perforates through a part of the lower end(downstream end) of the outer pipe 82 that is a closed end”, means thata part of the lower end (downstream end) of the outer pipe 82 that is aclosed end, is provided with an opening into which the lower end(downstream end) of the outer pipe 81 that is an open end is fit.

The lower end (downstream end) (closed end) side of the outer pipe 82 isprovided with an opening 83. More specifically, a side wall on the lowerend (downstream end) (closed end) side of the outer pipe 82 is providedwith an opening 83 having a laterally elongated rectangular shape whoseside in the longitudinal direction of the outer pipe 82 is longer thanwhose side in the circumferential direction of the outer pipe 82. InFIG. 2, the opening 83 is located at approximately the same height asthat of the opening of the returning pipe 11 provided in the sideportion (side wall) of the downstream side pit 10. Here, the opening 83is preferably located at approximately the same height as that of theopening of the return pipe 11 provided in the side portion (side wall)of the downstream side pit, or the upper end (upstream side end) of theopening 83 is preferably located at a position lower than the upper end(upstream side end) of the opening of the return pipe 11.

When the lower end (downstream end) side of the extension pipe 8 has adouble pipe structure, the double pipe structure functions as aseparating mechanism for separating from a main flow of a molten glass Gmoving from the downfalling pipe 5 to the downstream side pit 10, aportion containing many bubbles due to generation of bubbles on theinterface between molten glass and a wall face of the conduit for moltenglass or caused by lowering of vacuum degassing effect due to rise ofmolten glass level in the vacuum degassing vessel. The reason that theextension pipe 8 functions as a separating mechanism, will be describedas follows.

As described above, one reason why the bubbles in the molten glassincreases in spite of the vacuum degassing, is generation of bubbles onthe interface between molten glass and the wall face of the conduit formolten glass. Bubbles generated on the interface between molten glassand the wall face of molten glass, are not uniformly dispersed in themolten glass, but they flow in a boundary laminar flow having a certainthickness along the wall face of the conduit, for example, in a boundarylaminar flow having a thickness of from about 10 to 50 mm.

Further, another reason why the bubbles in the molten glass increases,is because when the level of molten glass in the vacuum degassing vesselrises, the vacuum degassing effect decreases to prevent moving up ofbubbles present in the vicinity of the bottom face of the vacuumdegassing vessel. Such bubbles present in the vicinity of the bottomface of the vacuum degassing vessel 3 flow out from the vacuum degassingvessel 3 to reach the downfalling pipe 5 (further to the extension pipe8), and they flow in a boundary laminar flow having a thickness of, forexample, about 3 to 5 mm formed along a wall face of the downfallingpipe 5 (further of the extension pipe 8), more specifically, along awall face on a side of the pipes close to the upstream side in thehorizontal flow direction of the molten glass in the vacuum degassingvessel (hereinafter referred to as “upstream side in horizontaldirection”).

Hereinafter, in this specification, a “boundary laminar flow” includes aboundary laminar flow caused by generation of bubbles on the interfacebetween molten glass and a wall face of the conduit for molten glass,and a boundary laminar flow caused by decrease of degassing effect dueto rise of the level of molten glass in the vacuum degassing vessel.

When a molten glass flow containing such a boundary laminar flow reachesthe double pipe structure of the extension pipe shown in FIG. 2, theboundary laminar flow containing many bubbles moves into a gap portion(hereinafter referred to also as “gap portion of double pipe structure”)between the outer wall of the inner pipe 81 and the inner wall of theouter pipe 82. Meanwhile, molten glass of a main flow other than theboundary laminar flow (hereinafter referred to as “main flow”) movesinto a gap inside the inner pipe 81 (hereinafter referred to as “insideinner pipe 81”). Accordingly, the boundary laminar flow is physicallysplit from the main flow. Here, the main flow means a molten glass flowwherein bubbles are sufficiently removed by a vacuum degassing, whichcan be finally used as a product.

The main flow moving inside the inner pipe 81 moves in the direction ofan arrow A in the Figure. Namely, the main flow passes through the lowerend (downstream end) (open end) of the inner pipe 81, and moves in thedownstream direction in the downstream side pit 10. Meanwhile, theboundary laminar flow moving in the gap portion of the double pipestructure moves in the direction of an arrow B in the Figure. Namely,the boundary laminar flow flows out through the opening 83 provided onthe side wall of the outer pipe 82 into the downstream side pit 10, andmoves through the opening provided in the side portion (side wall) ofthe downstream side pit 10 into the returning pipe 11.

As a result, only the main flow from which bubbles are sufficientlyremoved by the vacuum degassing is supplied to a forming apparatus.

On the other hand, the boundary laminar flow containing many bubblesmoves in the returning pipe 11 and is sent to the upstream side pit 9.The boundary laminar flow that has reached the upstream side pit 9,moves up in the uprising pipe 4 (more specifically, the extension pipe 7and the uprising pipe 4) together with molten glass newly supplied froma melting vessel 100, and is sent to the vacuum degassing vessel 3.

Thus, in the vacuum degassing apparatus of the present invention, sincethe boundary laminar flow containing many bubbles is sent to the vacuumdegassing vessel 3 and subjected to vacuum degassing again, it ispossible to suppress the influence of generation of bubbles on theinterface between molten glass and a wall face of the conduit for moltenglass or lowering of vacuum degassing effect due to rise of the level ofmolten glass in the vacuum degassing vessel.

Moreover, the boundary laminar flow sent to the upstream side pit 9rises along the returning pipe side of the uprising pipe 4 as it isalong with molten glass supplied from the melting vessel 100.Accordingly, in the vacuum degassing vessel 3, it is considered that themolten glass separated from the main flow and returned to the vacuumdegassing vessel 3 constitutes a lower flow while the molten glass newlysupplied from the melting vessel 100 constitutes an upper flow, to forma two-layer flow. Since such a two-layer flow is formed, a virtual depthof the molten glass newly supplied from the melting vessel 100 in thevacuum degassing vessel 3, decreases. Accordingly, improvement of vacuumdegassing effect is expected.

Here, the above effect obtainable by separating the boundary laminarflow containing many bubbles by the separating mechanism and returningit through the returning pipe to the vacuum degassing vessel, is alsoexhibited in the same manner in the vacuum degassing apparatuses ofsecond to fourth embodiments.

In the first embodiment of the vacuum degassing apparatus of the presentinvention, the following points should be noted to properly separate theboundary laminar flow from the main flow. Please refer to FIG. 3 for thefollowing points. Here, FIG. 3 is the same as FIG. 2 except that symbolsshowing dimensions of portions are added in FIG. 3.

In the extension pipe 8 shown in FIG. 3, in order to prevent theboundary laminar flow (represented by an arrow B in FIG. 2) from theopening 83 from merging again with the main flow of molten glass(represented by an arrow A in FIG. 2) from the inner pipe 81, the innerpipe 83 shown in FIG. 3 preferably protrude from the lower end(downstream end) (closed end) of the outer pipe 82.

Although the situation depends on the position and shape of the opening83 provided in the side wall of the outer pipe 82, if the distancebetween the opening 83 being an outlet of the boundary laminar flow andthe lower end (downstream end) of the inner pipe 81 being an outlet ofthe main flow, is small, the boundary laminar flow and the main flow,that are separated by the double pipe mechanism, may merge again. Whenthe inner pipe 81 protrudes from the lower end (downstream end) (closedend) of the outer pipe 82, the lower end (downstream end) of the innerpipe 81 is sufficiently distant from the opening 83. Accordingly, thereis no risk that the boundary laminar flow merges with the main flowagain, and it is possible to securely separate these flows.

In order to securely separate the boundary laminar flow from the mainflow, the distance L_(exit) from the lower end (downstream side end) ofthe opening 83 to the lower end (downstream end) of the inner pipe 81,is preferably from 10 to 200 mm.

In order to physically separate the boundary laminar flow from the mainflow, the distance L_(in) (mm) from the upper end (upstream end) of theinner pipe 81 to the upper end (upstream side end) of the opening 83,and the inner diameter D_(in) (mm) of the inner pipe 81, preferablysatisfy the relation represented by the following formula (1).

L _(in) ≧D _(in)/2  (1)

When L_(in) and D_(in) satisfy the relation represented by the aboveformula (1), the length of the double pipe structure from the opening83, more specifically, the length of the gap portion in the double pipestructure from the opening 83, is sufficient for physically separatingthe boundary laminar flow from the main flow.

D_(in) changes depending on the size of the vacuum degassing apparatus,in particular, on the flow rate (t/day) of molten glass passing throughthe apparatus, but it is usually from 50 to 900 mm, more preferably from100 to 700 mm. L_(in) is preferably at least 50 mm, more preferably atleast 100 mm, particularly preferably at least 200 mm and at most 1,500mm. Here, the extension pipe 8 may have a double pipe structure in theentire length so long as there is no problem in the cost. On the otherhand, if L_(in) is 50 mm or less, the distance to the opening 83 becomesinsufficient and separation of the boundary laminar flow from the mainflow may become insufficient.

In the first embodiment of the vacuum degassing apparatus of the presentinvention, L_(in) (mm) and D_(in) (mm) preferably satisfy the relationrepresented by the following formula (2), more preferably satisfy therelation represented by the following formula (3).

L _(in)≧1.0×D _(in)  (2)

1.0×D _(in) ≦L _(in)≦4×D _(in)  (3)

In the vacuum degassing apparatus 1 shown in FIG. 1, the length of theentire extension pipe 8 including a portion other than the double pipestructure, is usually from 100 to 3,000 mm, more preferably from 200 to1,500 mm. In the vacuum degassing apparatus 1 having a structure shownin FIG. 1, in order to adjust the level of molten glass G in the vacuumdegassing vessel 3, it is necessary to move up and down the vacuumdegassing vessel 3 by maximum about 600 mm. At this time, it isnecessary that the leading edge of the extension pipe 8 is alwaysimmersed in molten glass G in the downstream side pit 10. When thelength of the entire extension pipe 8 is within the above range, even ifthe vacuum degassing vessel 3 is moved up and down in the maximum range,the leading edge of the extension pipe 8 is always immersed in moltenglass G in the downstream side pit 10.

In order to physically separate the boundary laminar flow from the mainflow, the difference D_(out-in) (mm) between the inner diameter of theouter pipe 82 and the outer diameter of the inner pipe 81 preferablysatisfies the relation represented by the following formula (4) with theinner diameter D_(in) (mm) of the inner pipe 81. Here, D_(out-in)/2corresponds to the width of the gap portion of the double pipestructure.

D _(out-in)/2≧0.02×D _(in)  (4)

When D_(out-in) and D_(in) satisfy the relation represented by the aboveformula (4), the width of the gap portion of the double pipe structureis sufficient for physically separating the boundary laminar flow fromthe main flow.

The thickness of the boundary laminar flow slightly changes depending onthe temperature or the viscosity of molten glass or the materialconstituting the flow path, etc., and it is usually about from 3 to 5mm. In order to prevent such a boundary laminar flow from flowing intothe main flow, the above relation is required.

D_(out-in)/2 is, specifically, preferably at least 5 mm, more preferablyat least 10 mm, particularly preferably at most 100 mm. If D_(out-in)/2exceeds 100 mm, the width of the gap portion of the double pipestructure becomes too large in relation to the thickness of the boundarylaminar flow, and accordingly, the amount of the molten glass separatedfrom the main flow and moving into the gap portion of the double pipestructure increases, to decrease the yield of production of the glass,such being not preferred.

In the extension pipe 8 shown in FIG. 2, it is preferred that only theboundary laminar flow is separated to move into the gap portion of thedouble pipe structure, and in order to achieve this, it is ideal to makethe width of the gap portion of the double pipe structure to besubstantially the same as the thickness of the boundary laminar flow.However, the thickness of the boundary laminar flow at a time ofconducting a vacuum degassing is not always constant and it may vary.Accordingly, in order to securely separate the boundary laminar flow tomove it to the gap portion of the double pipe structure, the width ofthe gap portion of the double pipe structure is preferably larger thanthe thickness of the boundary laminar flow to a certain extent. In thiscase, a part of the main flow is also separated to move into the gapportion of the double pipe structure.

Accordingly, if the width of the gap portion of the double pipestructure is too large as compared with the thickness of the boundarylaminar flow, the amount of main flow separated to move into the gapportion of the double pipe structure increases to lower the yield ofproduction of glass, such being not preferred.

In the first embodiment of the vacuum degassing apparatus of the presentinvention, D_(out-in) (mm) and D_(in) (mm) preferably satisfy therelation represented by the following formula (5), more preferablysatisfy the relation represented by the following formula (6).

D _(out-in)/2≧0.04×D _(in)  (5)

0.04×D _(in) ≦D _(out-in)/2≦0.25×D _(in)  (6)

Here, D_(in) is usually from 50 to 900 mm as described above, morepreferably from 100 to 700 mm. The wall thicknesses of the inner pipe 81and the outer pipe 82 that are to be employed for a conduit structure ofmolten glass and made of platinum or a platinum alloy, are eachpreferably from 0.4 to 6 mm, more preferably from 0.8 to 4 mm.

For the above reasons, the outer diameter of the inner pipe 81 ispreferably from 55 to 905 mm, more preferably from 105 to 705 mm. Theouter diameter of the outer pipe 82 is preferably from 70 to 1,200 mm,more preferably from 100 to 1,000 mm.

Further, in order to physically separate the boundary laminar flow fromthe main flow, the distance L_(in) (mm) from the upper end (upstreamend) of the inner pipe 81 to the upper end (upstream side end) of theopening 83, and the difference D_(out-in) (mm) between the innerdiameter of the outer pipe 82 and the outer diameter of the inner pipe81, preferably satisfy the relation represented by the following formula(7).

L _(in)≧(D _(out-in)/2)×3  (7)

When L_(in) and D_(out-in) satisfy the above relation, the length L_(in)of the gap portion of the double pipe structure from the opening 83 inrelation to the width (D_(our-in)/2) of the gap portion of the doublepipe structure, is sufficient for physically separating the boundarylaminar flow from the main flow.

Further, D_(out-in)×20≧L_(in) is preferably satisfied.

Further, in order to physically separate the boundary laminar flow fromthe main flow, the cross-sectional area difference S_(out-in) (mm²)obtained by subtracting the cross-sectional area of the flow path in theinner pipe 81 from the cross-sectional area of the flow path in theouter pipe 82, and the cross-sectional area S_(in) (mm²) of the innerpipe 81, preferably satisfy the relation represented by the followingformula (8).

S _(out-in) ≦S _(in)  (8)

Here, the cross-sectional areas of the flow paths in the outer pipe 82and the inner pipe 81, mean cross-sectional areas perpendicular to thelongitudinal directions of the flow paths in the outer pipe 82 and theinner pipe 81. When S_(out-in) and S_(in) satisfy the relationrepresented by the formula (8), the width of the gap portion of thedouble pipe structure does not become too large in relation to thethickness of the boundary laminar flow, and accordingly, the amount ofmolten glass in the main flow separated to move into the gap portion ofthe double pipe structure does not increase. Accordingly, the yield ofproduction of glass does not decrease.

In the first embodiment of the vacuum degassing apparatus of the presentinvention, S_(out-in) (mm²) and S_(in) (mm²) preferably satisfy therelation represented by the following formula (9), more preferablysatisfy the relation represented by the following formula (10).

S _(out-in)≦0.90×S _(in)  (9)

S _(out-in)≦0.80×S _(in)  (10)

Further, 0.50×S_(in)≦S_(out-in) is preferably satisfied.

Further, in order to physically separate the boundary laminar flow fromthe main flow, the area S₈₃ (mm²) of the opening 83 and the innerdiameter D_(out) (mm) of the outer pipe 82 preferably satisfy therelation represented by the following formula (11).

S ₈₃≧9×D _(out)  (11)

Here, the area S₈₃ of the opening 83 is a projected area of the opening83 on a plain. When S₈₃ and D_(out) satisfy the relation represented bythe above formula (11), the opening 83 is large enough to flow outmolten glass passing through the gap portion between the outer pipe 82and the inner pipe 81, and accordingly, the flow resistance of theboundary laminar flow does not increase significantly when it passesthrough the opening 83. If the opening 83 is extremely small, the flowresistance of the boundary laminar flow significantly increases when itpasses through the opening 83. As a result, between the boundary laminarflow moving through the gap portion of the double pipe structure and themain flow moving inside the inner pipe 81, a significant difference offluidity occurs to decrease the effect of separating the boundarylaminar flow from the main flow. Here, the above formula (11) is aformula established from the viewpoint that flow of molten glass of atleast 3 mm needs to be flown out through the opening when the thicknessof the boundary laminar flow is 3 mm.

In the first embodiment of the vacuum degassing apparatus of the presentinvention, S₈₃ (mm²) and D_(out) (mm) preferably satisfy the relationrepresented by the following formula (12), more preferably satisfy therelation represented by the following formula (13).

S ₈₃≧12×D _(out)  (12)

20×D _(out) S ₈₃≦90×D _(out)  (13)

When S₈₃ is larger than 90×D_(out), the size of the opening 83 becomestoo large in relation to the inner diameter of the outer pipe 82, andaccordingly, the boundary laminar flow separated by the double pipestructure may merge with the main flow again.

Here, the opening 83 is preferably provided in the vicinity of theclosed end of the outer pipe 82. Here, the vicinity of the closed endincludes not only a portion of the closed end of the outer pipe 82, butalso a side wall portion of the outer pipe 82 close to the closed end asshown in FIG. 2. “Portion close to closed end” here means a portion ofthe outer pipe 82 within 200 mm from the closed end.

By providing the opening 83 in the vicinity of the closed end, it ispossible to increase the length of the gap portion in the double pipestructure for physically separating the boundary laminar flow from themain flow.

Further, the number of the opening 83 is not necessarily one but it maybe plural. When the number of the openings is plural, it is sufficientthat at least one opening is located at a position within 200 mm fromthe closed end of the outer pipe 82.

Further, when the opening 83 has a rectangular shape, it is preferablynot a rectangular shape elongated in the longitudinal direction of theouter pipe 82 (that is a vertically elongated rectangular shape) but arectangular shape elongated in the circumferential direction of theouter pipe 82 (that is a laterally elongated rectangular shape) for thereason that the flow resistance is small when the boundary laminar flowpasses through the opening 83.

The shape of the opening 83 is not limited to a rectangular shape, butit may be a different shape. For example, it may be a square, circularor elliptical shape. Further, it may be a polygonal shape such as atriangular, pentangular, hexagonal or octagonal shape.

The length of the opening 83 in the circumferential direction of theouter pipe 82 (that is the width of the opening 83) is preferablysmaller than the width of the opening of the returning pipe 11 providedin the side portion (side wall) of the downstream side pit 10. If thewidth of the opening 83 is larger than the width of the opening of thereturning pipe 11, the boundary laminar flow separated by the doublepipe structure may merge with the main flow again.

Here, the width of the opening 83 is a width of a shape produced byprojecting the opening 83 on a plane.

Likewise, when the opening of the returning pipe 11 has a curved planeshape, the width of the opening of the returning pipe 11 is a width of ashape obtained by projecting the opening on a plane.

In FIG. 2, the opening 83 provided in the outer pipe 82 is located at aposition close to the opening of the returning pipe 11 provided in aside portion (side wall) of the downstream side pit 10, morespecifically, located at the same height of the opening of the returningpipe 11. Here, the upper end (upstream side end) of the opening 83 ispreferably located at a position lower than the upper end (upstream sideend) of the opening of the returning pipe 11 (specifically, the upperend (upstream side end) of the opening 83 is preferably located at aposition from 0 to 500 mm lower than the upper end (upstream side end)of the opening of the returning pipe 11). In order to prevent theboundary laminar flow and the main flow separated by the double pipestructure from merging again, the upper end (upstream side end) of theopening 83 is preferably located at a position lower than the upper end(upstream side end) of the opening of the returning pipe 11.

As described above, in the vacuum degassing apparatus 1 having astructure shown in FIG. 1, in order to adjust the level of the moltenglass in the vacuum degassing vessel 3, it is necessary to move up anddown the vacuum degassing vessel 3 by maximum about 600 mm. Accordingly,the positional relationship between the opening 83 and the opening ofthe returning pipe 11 provided in a side portion (side wall) of thedownstream side pit 10, changes to a certain extent from the positionalrelationship shown in FIG. 2. However, it is preferred that, even whenthe vacuum degassing vessel 3 is moved up and down, the opening 83 doesnot leave too far from the opening of the returning pipe 11 for thepurpose of preventing the boundary laminar flow separated by the doublepipe structure from merging with the main flow again. The distancebetween the upper end of the opening 83 and the upper end (upstream sideend) of the opening of the returning pipe 11, in a state that they areat a maximum distance, is preferably at most 400 mm, more preferably atmost 200 mm.

Further, in order to prevent the boundary laminar flow flown out fromthe opening 83 from merging with the main flow again, the area of theopening of the returning pipe 11 provided in a side portion (side wall)of the downstream side pit 10 needs to be large to a certain extent.Specifically, provided that the area of the opening of the returningpipe 11 is S₁₁ (mm²), it preferably satisfies the relation representedby the following formula (14) with the area S₈₃ (mm²) of the opening 83.

S ₁₁ ≧S ₈₃  (14)

In the first embodiment of the vacuum degassing apparatus of the presentinvention, the inner pipe 81 and the outer pipe 82 constituting thedouble pipe structure, are hollow pipes made of platinum or a platinumalloy, and their shapes are not limited so long as they satisfy thefollowing conditions (1) to (3).

(1) The inner pipe 81 has an upper end (upstream end) and a lower end(downstream end) that are open ends.

(2) The outer pipe 82 has an upper end (upstream end) that is an openend, and a lower end (downstream end) that is a closed end. Here, theinner pipe 81 perforates through a part of the lower end (downstreamend) of the outer pipe 82 that is a closed end.

(3) An opening 83 is provided in a lower end (downstream end) side ofthe outer pipe 82.

Accordingly, the inner pipe 81 and the outer pipe 82 may have across-section of an elliptical shape, or a polygonal shape such as arectangular, hexagonal or octagonal shape.

Further, in the extension pipe 8 shown in FIG. 2, the closed end (lowerend (downstream end)) of the outer pipe 82 is a horizontal end. However,the shape of the closed end of the outer pipe is not limited thereto.FIG. 4 is a cross-sectional view showing another embodiment of theextension pipe, wherein the shape of the closed end of the outer pipe isdifferent from that of the extension pipe 8 shown in FIG. 2. Theextension pipe 8′ shown in FIG. 4 is the same as the extension pipeshown in FIG. 2 in that the inner pipe 81′ and the outer pipe 82′ eachconstitutes a double pipe structure, but they are different in that theclosed end (lower end (downstream end)) of the outer pipe 82′ has ashape that it is obliquely inclined.

More specifically, with respect to the length of the outer pipe 82′, thelength on a side of the pipe facing to the opening of the returning pipe11 provided in a side portion (side wall) of the downstream side pit 10is longer than that on the other side of the pipe, whereby the closedend (lower end (downstream end)) of the outer pipe 82′ is obliquelyinclined.

An opening 83′ is provided in a side wall of the outer pipe 82′ on aside of the pipe facing to the opening of the returning pipe 11 in thevicinity of the lower end (downstream end). The extension pipe 8′ shownin FIG. 4 can guide a boundary laminar flow, that is moving through thegap portion of the double pipe structure, along the closed end (lowerend (downstream end)) of the outer pipe 82′, that is obliquely inclined,to the direction of the opening 83′.

Here, the open end of the inner pipe 81′, that is the upper end(upstream side end) or the lower end (downstream side end), may have ashape that is obliquely inclined. For example, in FIG. 4, when the innerpipe 81′ has a shape that the upper end (upstream end) of the inner pipe81′ on its side far from the opening 83′ is lower than the upper end(upstream end) of the inner pipe 81′ on its side close to the opening83′, the following effect is exhibited. With respect to the movingdistance of the boundary laminar flow in the double pipe structure toreach the opening 83′, when the upper end (upstream end) of the innerpipe 81′ is not inclined, a moving distance of the boundary laminar flowmoving through a gap portion on the side of the pipe far from theopening 83′, is longer than a moving distance through a gap portion onthe side of the pipe close to the opening 83′, and accordingly, apressure loss of the boundary laminar flow moving through the gapportion may be formed. When the shape is inclined so that the upper end(upstream end) of the inner pipe 81′ on its side far from the opening83′ is lower than the upper end (upstream end) of the inner pipe 81′ onits side close to the opening 83′, then, the difference between themoving distance of the boundary laminar flow through a gap portion onthe side of the pipe far from the opening 83′, and the moving distanceof the boundary laminar flow through a gap portion on the side of thepipe close to the opening 83′, becomes small, and accordingly, thepossibility of forming a pressure loss of the laminar flow movingthrough the gap portion, becomes small.

The relations of the above formulae (1) to (14) are applicable also tothe extension pipe 8′ shown in FIG. 4. Here, in the extension pipe 8′shown in FIG. 4, the distance L_(exit) (refer to FIG. 3) from the lowerend (downstream side end) of the opening 83 to the lower end (downstreamend) of the inner pipe 81, is the distance from the lower end(downstream side end) of the opening 83′ to the lower end (downstreamend) of the inner pipe 81′. The distance L_(in) from the upper end(upstream end) of the inner pipe 81′ to the upper end (upstream sideend) of the opening 83′, the inner diameter D_(in) of the inner pipe81′, the distance D_(out-in) between the inner diameter of the outerpipe 82′ and the outer diameter of the inner pipe 81′, thecross-sectional areas of the flow paths in the inner pipe 81′ and theouter pipe 82′, the area S₈₃ of the opening 83′ and the area S₁₁ of theopening of the returning pipe 11 provided in a side portion (side wall)of the downstream side pit 10, are defined in the same manner as thoseof the extension pipe 8 shown in FIG. 2.

In the extension pipes 8, 8′ shown in FIGS. 2 to 4, the inner pipes 81,81′ are each a hollow cylindrical pipe having a straight pipe shape witha constant diameter (inner diameter, outer diameter) in the entirelength, but the shape of the inner pipe is not limited thereto. FIG. 5is a cross-sectional view showing another embodiment of the extensionpipe, wherein the shape of the inner pipe is different from that in theextension pipes 8, 8′ shown in FIGS. 2 to 4. In extension pipes 8, 8′shown in FIG. 5, an inner pipe 81″ and an outer pipe 82″ constitute adouble pipe structure in the same manner as the extension pipe 8, 8′shown in FIGS. 2 to 4. However, in the extension pipe 8″ shown in FIG.5, the inner pipe 81″ has a part (the lower end (upstream end) vicinityportion in the Figure) wherein the diameter expands to form a taper pipeshape. The lower end (downstream end) of the inner pipe 81″ having ataper pipe shape is joined with an inner wall of the outer pipe 82″ soas to form a closed end at a lower end (downstream end) of a gap portionbetween the outer wall of the inner pipe 81″ and the inner wall of theouter pipe 82″. Accordingly, the lower end (downstream end) of the innerpipe 81″ does not protrude from the closed end of the outer pipe 82″. Inthe extension pipe 8″ shown in FIG. 5, it is possible to guide theboundary laminar flow moving through the gap portion of the double pipestructure, along the outer wall of the inner pipe 81″ having a taperpipe shape, to the direction of the opening 83″.

The relations of the above formulae (1) to (14) are applicable to theextension pipe 8″ shown in FIG. 5. Here, in the extension pipe 8″ shownin FIG. 5, the inner diameter D_(in) of the inner pipe 81″ is an innerdiameter of a portion of the inner pipe 81″ whose diameter is notexpanded. The distance L_(in) from the upper end (upstream end) of theinner pipe 81″ to the upper end (upstream side end) of the opening 83″,the difference D_(out-in) between the inner diameter of the outer pipe82″ and the outer diameter of the inner pipe 81″, the cross-sectionalareas of the flow paths in the inner pipe 81″ and the outer pipe 82″,the area S₈₃ of the opening 83″ and the area S₁₁ of the opening of thereturning pipe 11 provided in a side portion (side wall) of thedownstream side pit 10, are defined in the same manner as those of theextension pipe 8 shown in FIG. 2.

Further, in the extension pipe 8, 8′ or 8″ shown in FIGS. 2 to 5, theopening 83, 83′ or 83″ provided in a lower end (downstream end) side ofthe outer pipe 82, 82′ or 82″ are each positioned closely to the openingof the returning pipe 11 provided in a side portion (side wall) of thedownstream side pit 10, to prevent the boundary laminar flow flown outfrom the opening 83, 83′ or 83″ from merging again with the main flow.However, as shown in the extension pipe 8′″ shown in FIG. 6, a conduitpipe 84 for guiding the boundary laminar flow flown out from the opening83′″ directly to the opening of the returning pipe 11, may be provided.Here, in the extension pipe 8′″ shown in FIG. 6, the opening 83′″ is notprovided in a side wall of the outer pipe 82′″ but provided in a part ofa closed end.

The extension pipe 8′″ shown in FIG. 6 has a complicated structure, butis excellent in that it can securely separate the boundary laminar flowfrom the main flow.

In the extension pipe 8, 8′ or 8″ shown in FIGS. 2 to 5, a singleopening portion 83, 83′ or 83″ is provided in a side wall in thevicinity of the lower end (downstream end) of the outer pipe 82, 82′ or82″, and in the extension pipe 8′″ shown in FIG. 6, a single opening83′″ is provided in a part of a closed end of the outer pipe 82′″.However, the number of openings is not limited thereto, and it may beplural. In this case, the plurality of openings may be provided so as tobe arranged at positions of the same height in the outer pipe (that isarranged in a left to right direction), or they may be provided at thesame position in the circumferential direction at different heights inthe outer pipe (namely, they are vertically arranged). Further, they maybe provided in a combined form of these two embodiments (namely, theyare arranged in vertical and lateral directions).

Here, when a plurality of openings are provided, L_(exit) is defined asa distance from the lower end (downstream side end) of an openinglocated at the lowest position, to the lower end (downstream end) of theinner pipe. L_(in) is defined as a distance from the upper end (upstreamside end) of an opening located at the highest position to the upper end(upstream end) of the inner pipe. S is defined as the total area of allopenings. Here, the above formula (14) is applied to the openingscorresponding to each other (that are the opening on the lower end(downstream side end) side of the outer pipe and the opening in thedrain out).

The molten glass (boundary laminar flow) separated by the double pipestructure of the extension pipe 8 by the above principle, is returnedthrough the returning pipe 11 to the upstream side pit 9.

The returning pipe 11 is a hollow pipe made of refractory bricks,platinum or a platinum alloy. When the returning pipe 11 is a hollowpipe made of refractory bricks, it is preferably a hollow pipe made ofrefractory bricks having a rectangular cross section, and the internalshape forming a flow path of molten glass preferably has a rectangularcross section or a circular cross section. Meanwhile, when the returningpipe 11 is a hollow pipe made of platinum or a platinum alloy, the innercross-sectional shape forming a flow path of molten glass, is preferablya circular or elliptical shape. In any of these cases, in the returningpipe 11, the internal shape forming the flow path of molten glasspreferably agrees with the shape of the opening provided in a side faceof the downstream side pit 10, for the purpose of preventing slack ofthe molten glass. Further, in order to prevent increase of flowresistance or generation of pressure loss of molten glass, thecross-sectional area of the returning pipe 11 is preferably constantthrough the entire length of the returning pipe 11. Accordingly, thecross-sectional area of the returning pipe 11 is preferablysubstantially the same as the area of the opening provided in a sideportion (side wall) of the downstream side pit 10 and the area of theopening provided in a side portion (side wall) of the upstream side pit9.

Further, the returning pipe 11 is preferably provided so that the pathlength to the upstream side pit 9 is minimized. For this reason, thereturning pipe 11 preferably extends in a horizontal direction towardsthe upstream side pit 9. Further, in order to prevent increase of flowresistance of molten glass in the returning pipe 11, it is preferred tominimize curved portions provided in the returning pipe 11. In FIG. 1,the returning pipe 11 extends upwardly at a portion where a pumpingmeans 12 is provided, and the returning pipe 11 extends downwardly at aportion where a stirring means 13 is provided. However, the positions ofthe pumping means 12 and the stirring means 13 may be reversed, andtheir positions are not limited so long as their functions are obtained.

The dimensions of the returning pipe 11 are appropriately selecteddepending on a vacuum degassing apparatus to be used. In the case of thereturning pipe 11 shown in FIG. 1, a specific example of the dimensionsis as follows.

Length in horizontal direction: from 1 to 15 m, preferably from 1 to 12m, more preferably from 1 to 10 m

Width of internal cross-sectional shape: from 0.2 to 7 m, preferablyfrom 0.2 to 5 m, more preferably from 0.2 to 3 m

The vacuum degassing apparatus shown in FIG. 1 has a pumping means 12for controlling the flow of molten glass in the returning pipe 11. Thepumping means 12 controls the flow of molten glass in the returning pipe11, to form a molten glass flow g of a constant flow rate toward theupstream direction (represented by an arrow). By this construction,slack of molten glass in the returning pipe 11 is prevented. Further, itis possible to prevent backflow of molten glass in the returning pipe 11due to entry of molten glass from the upstream side pit 9 into thereturning pipe 11. Further, by the pumping means 12, it is possible toadjust the flow rate of molten glass in the returning pipe 11.

Here, if it is possible to control the molten glass flow in thereturning pipe 11 to form a molten glass flow of a constant flow ratetoward the upstream direction (indicated by an arrow) without employingthe pumping means 12, it is not necessary to use the pumping means. Forexample, when the difference between the temperature of molten glass inthe extension pipe 7 connected to the uprising pipe 4 and thetemperature of the molten glass in the extension pipe 8 connected to thedownfalling pipe 5, is large, even without employing the pumping means12, a molten glass flow toward the upstream direction (indicated by anarrow) is formed in the returning pipe 11 by the effect of thermalconvection.

The pumping means 12 is not particularly limited so long as it has aheat resistance durable against a high temperature molten glass flow andusable for a molten glass having a high viscosity, and it may be widelyselectable from pumping means having a known structure. Among these, anaxial flow type pump is preferred for the reason that it has a highdurability against high temperature. As the axial flow type pump, onehaving propeller-shaped blades is widely known, and an axial flow typepump having a spiral blade is particularly preferred since it provides ahigh efficiency.

In FIG. 1, a pumping means 12 is provided in the vicinity of the centerof the returning pipe 11, but the position to provide the pumping meansis not particularly limited, and it may be provided at a position closerto the downstream side pit 11, or a position closer to the upstream sidepit 9. Further, so long as the molten glass flow in the returning pipe11 can be properly controlled, the pumping means may be provided in thedownstream side pit 10, more specifically, in the vicinity of theopening of the returning pipe 11 in the downstream side pit 10, or inthe upstream side pit 9, for example, in the vicinity of the opening ofthe returning pipe 11 in the upstream side pit 9.

Further, in FIG. 1, a single pumping means 12 is provided in thereturning pipe 11, but the number of pumping means 12 is not limitedthereto, and a plurality of pumping means may be provided. For example,instead of the stirring means 13 in FIG. 1, an axial flow type pump maybe provided as a pumping means.

The vacuum degassing apparatus shown in FIG. 1 has a stirring means 13for stirring molten glass passing through the returning pipe 11. Thestirring means 13 is not an essential constituent, but in order toincrease the homogeneity of molten glass returned through the returningpipe 11 to the upstream side pit 9, the stirring means is preferablyprovided. The stirring means may be widely selectable from known meansused for the purpose of stirring molten glass.

Here, as described above, the axial flow type pump has a function ofstirring molten glass. When it is possible to sufficiently increase thehomogeneity of molten glass by a stirring function of an axial flow typepump provided as the pumping means, an independent stirring means is notnecessarily provided.

Further, in the molten glass passing through a conduit structure likethe returning pipe 11 provided in the horizontal direction, unevennessof temperature of molten glass may be formed. For example, a moltenglass on the bottom side of the returning pipe 11 may have a lowertemperature than that of the molten glass on the upper side in somecases. When such an uneven temperature is formed, homogeneity of themolten glass is deteriorated, such being not preferred.

In the first embodiment of the vacuum degassing apparatus of the presentinvention, in order to prevent formation of an uneven temperature ofmolten glass passing through the returning pipe 11, a heating means forheating molten glass passing through the returning pipe 11, for example,a means for heating the molten glass from the bottom side of thereturning pipe 11, may be provided. When such a heating means isprovided, the type of the heating means is not particularly limited, aheating means similar to one used for heating a glass in a glass meltingvessel, may be employed. Namely, a means for heating molten glass bycombusting a fuel or a means of heating molten glass by using electricpower, etc. may be employed.

The second embodiment of the vacuum degassing apparatus of the presentinvention is the same as the first embodiment of the vacuum degassingapparatus of the present invention except that no extension pipe isattached to a lower end (downstream end) of the downstream pipe, and thedownfalling pipe itself is a hollow pipe made of platinum or a platinumalloy having a double pipe structure on its lower end (downstream end)side. Accordingly, in the second embodiment of the vacuum degassingapparatus of the present invention, the lower end (downstream end) ofthe downfalling pipe is fit into the open end of the downstream sidepit, and is immersed in molten glass in the downstream side pit.

In the second embodiment of the vacuum degassing apparatus of thepresent invention, the double pipe structure of the downfalling pipefunctions as a separating mechanism for separating a boundary laminarflow containing many bubbles from a main flow of molten glass movingfrom the downfalling pipe to the downstream side pit.

Here, features to be satisfied by the downfalling pipe having a doublepipe structure, are similar to those described with respect to thedouble pipe structure of the extension pipe in the first embodiment ofthe vacuum degassing apparatus of the present invention.

Next, a third embodiment of the vacuum degassing apparatus of thepresent invention will be described.

FIG. 7 is a cross-sectional view showing the third embodiment of thevacuum degassing apparatus of the present invention. The vacuumdegassing apparatus 1′ shown in FIG. 7 is the same as the vacuumdegassing apparatus 1 shown in FIG. 1 except for the followingdifferences.

-   -   An extension pipe 14 connected to a lower end side (downstream        end) of the downfalling pipe 15 does not have a double pipe        structure.    -   A downstream side pit 15 has a structure to be described later.

FIG. 8 is a partial enlarged view showing the downstream side pit 15 andits vicinity of the vacuum degassing apparatus 1′ shown in FIG. 7. Thedownstream side pit 15 shown in FIG. 8 has a double pipe structureconstituted by a pit main body 16 forming an outer pipe, an inner pipe17 located in the pit main body 16 and extending in the downstreamdirection. This double pipe structure functions as a separatingmechanism for separating a boundary laminar flow containing many bubblesfrom a main flow of molten glass G moving from the downfalling pipe 5 tothe downstream side pit 15.

The pit main body 16 has a cylindrical body having an open upper end(upstream end) and a closed bottom, wherein the shape of the opening ofthe upper end (upstream end) is, for example, a rectangular shape suchas a square shape or a circular shape. An opening of the returning pipe11 is provided in a side portion (side wall) of the pit main body 16.Here, the position to provide the opening of the returning pipe 11 isnot limited to the side portion (side wall) of the pit main body 16, butit may be provided in a bottom of the pit main body 16.

The inner pipe 17 is a hollow cylindrical pipe having open ends, and itscross-sectional shape is, for example, a circular shape. The inner pipe17 has one end located on the upstream side of the pipe along the flowdirection of molten glass, that is, on the downfalling pipe 5 side ofthe pipe, more specifically, on a side of the pipe close to theextension pipe 14 that is attached to a lower end (downstream end) ofthe downfalling pipe 5. The inner pipe 17 has the other end perforatingthrough a side portion (side wall) of the pit main body 16 and extendingin the downstream direction along the flow direction of molten glass.The entire shape of the inner pipe 17 is substantially an L shape.

The pit main body 16 and the inner pipe 17 are usually made of platinumor a platinum alloy. When the pit main body 16 and the inner pipe 17 aremade of platinum or a platinum alloy, their cross-sectional shapes areeach a circular or elliptical shape for the reason of easiness offabrication or hardness of deformation, etc.

Here, the pit main body 16 and the inner pipe 17 may be made ofrefractory bricks. When the pit main body 16 and the inner pipe 17 aremade of refractory bricks, their cross-sectional shapes are each apolygonal shape such as a rectangle, a circular or elliptical shape forthe reason of easiness of fabrication or prevention of corrosion ofrefractory bricks, etc.

In FIG. 8, the extension pipe 14 and the inner pipe 17 partially overlapwith each other. More specifically, the upper end (upstream end) of theinner pipe 17 is located inside the extension pipe 14, whereby theyoverlap with each other. However, it is not required that the extensionpipe 14 and the inner pipe 17 partially overlap, but it is acceptablethat they do not overlap with each other.

Further, in the vacuum degassing apparatus 1′ shown in FIG. 7, theextension pipe 14 made of platinum or a platinum alloy attached to alower end (downstream end) of a downfalling pipe 5 made of refractorybricks or platinum or a platinum alloy, is immersed in molten glass in adownstream side pit 15 (pit main body 16). However, a downfalling pipemade of platinum or a platinum alloy is immersed in molten glass in adownstream side pit in some vacuum degassing apparatuses. In such acase, the downfalling pipe made of platinum or a platinum alloy directlyoverlaps with an inner pipe of a downstream side pit. The thirdembodiment of the vacuum degassing apparatus of the present inventionincludes such a construction.

Hereinafter, in this specification, “a downstream pipe and an inner pipeof downstream side pit overlap” includes both of the following cases (a)and (b).

(a) An extension pipe made of platinum or a platinum alloy attached to alower end (downstream end) of a downfalling pipe made of refractorybricks, platinum or a platinum alloy, overlaps with an inner pipe of adownstream side pit.

(b) A downfalling pipe made of platinum or a platinum alloy directlyoverlaps with an inner pipe of a downstream side pit.

In the third embodiment of the vacuum degassing apparatus of the presentinvention, the following points should be noted to properly separate aboundary laminar flow from a main flow. Please refer to FIG. 9 for thefollowing points. Here, FIG. 9 is the same as FIG. 8 except that symbolsshowing dimensions of portions are added.

In FIG. 9, the inner diameter D₁ (mm) of the extension pipe 14 and theouter diameter D₂ (mm) of the inner pipe 17 preferably satisfy therelation represented by the following formula (15).

D ₁ >D ₂  (15)

Namely, in the third embodiment of the vacuum degassing apparatus of thepresent invention, when the downfalling pipe (including a case where itis an extension pipe) overlaps with an inner pipe of a downstream sidepit, they have a positional relationship that the upper end (upstreamend) of the inner pipe of the downstream side pit is located inside thedownfalling pipe.

In the third embodiment of the vacuum degassing apparatus of the presentinvention, when the extension pipe and the inner pipe have the abovepositional relationship, the following effects will be exhibited.

When a molten glass flow containing a boundary laminar flow reaches anoverlap portion of the extension pipe 14 and the inner pipe 17 in FIG.8, the boundary laminar flow containing many bubbles moves into a regionthat belongs to a gap between the inner wall of the extension pipe 14and the outer wall of the inner pipe 17, that is, a gap portion betweenthe inner wall of the extension pipe 14 and the outer wall of the innerpipe 17 (indicated by an arrow B in the Figure). Meanwhile, the mainflow other than the boundary laminar flow moves into the inner pipe 17(represented by an arrow A in the Figure). As a result, the boundarylaminar flow is physically separated from the main flow.

The main flow moving inside the inner pipe 17 moves along the directionof the arrow A in the Figure. Namely, it moves to the downstreamdirection in the inner pipe 17. Meanwhile, the boundary laminar flowmoving in the gap portion between the inner wall of the extension pipe14 and the outer wall of the inner pipe 17, moves along the direction ofthe arrow B in the Figure, and moves through an opening provided in aside portion (side wall) of the pit main body 16 into the returning pipe11.

Thus, the boundary laminar flow is physically separated from the mainflow, and only the main flow from which bubbles are sufficiently removedby vacuum degassing, is supplied to a forming apparatus. On the otherhand, the boundary laminar flow containing many bubbles moves in thereturning pipe 11, to be sent to an upstream side pit 9. The boundarylaminar flow that has reached the upstream side pit 9 moves up throughan uprising pipe 4 (more specifically, an extension pipe 7 and theuprising pipe 4) along with molten glass newly supplied from a meltingvessel 100, to be sent to a vacuum degassing vessel 3.

In order to physically separate the boundary laminar flow from the mainflow, the difference ΔD (mm) between the inner diameter D₁ (mm) of theextension pipe 14 and the outer diameter D₂ of the inner pipe 17,preferably satisfies the relation represented by the following formula(16) with the inner diameter D₃ (mm) of the inner pipe 17.

ΔD≧0.04×D ₃  (16)

When ΔD and D₃ satisfy the relation represented by the above formula(16), the width of a gap portion between the inner wall of the extensionpipe 14 and the outer wall of the inner pipe 17, that is ΔD/2, issufficient for physically separating the boundary laminar flow from themain flow.

ΔD is specifically preferably at least 10 mm, more preferably at least20 mm, particularly preferably at least 40 mm and at most 200 mm. If ΔDexceeds 200 mm, the width of the gap between the inner wall of theextension pipe 14 and the outer wall of the inner pipe 17 becomes toolarge with respect to the thickness of the boundary laminar flow, andthe flow rate of main flow decreases, such being not preferred.

In FIG. 8, it is preferred that only the boundary laminar flow isseparated to move to a gap portion between the inner wall of theextension pipe 14 and an outer wall of the inner pipe 17. In order toachieve this, it is ideal that the width of the gap portion between theinner wall of the extension pipe 14 and the outer wall of the inner pipe17 is substantially same as the thickness of the boundary laminar flow.

However, the thickness of the boundary laminar flow at a time ofcarrying out vacuum degassing, is not always constant and it may vary.Accordingly, in order to securely separate the boundary laminar flow tomove it into the gap portion between the inner wall of the extensionpipe 11 and the outer wall of the inner pipe 17, the width of the gapportion is preferably larger by a certain extent than the thickness ofthe boundary laminar flow. In this case, a part of main flow is alsoseparated and moves into the gap portion.

Accordingly, when the width of the gap portion between the inner wall ofthe extension pipe 14 and the outer wall of the inner pipe 17, isexcessively larger than the thickness of the boundary laminar flow, theamount of the main flow separated to move into the gap portion increasesto decrease the yield of the production of glass, such being notpreferred.

In the third embodiment of the vacuum degassing apparatus, ΔD (mm) andD₃ (mm) preferably satisfy the relation represented by the followingformula (17), more preferably satisfy the relation represented by thefollowing formula (18).

ΔD≧0.08×D ₃  (17)

0.1×D ₃ ≦ΔD≦0.6×D ₃  (18)

Here, D₃ is usually from 50 to 900 mm, more preferably from 100 to 700mm.

The wall thicknesses of the inner pipe 17 and the extension pipe 14,that are made of platinum or a platinum alloy, are preferably from 0.4to 6 mm, more preferably from 0.8 to 4 mm.

For the above reasons, the outer diameter D₂ of the inner pipe 17 ispreferably from 51 to 912 mm, more preferably from 102 to 708 mm. Theouter diameter of the extension pipe 14 is preferably from 60 to 1,300mm, more preferably from 123 to 1,000 mm.

Further, in order to physically separate the boundary laminar flow fromthe main flow, the cross-sectional area difference ΔS (mm²) obtained bysubtracting the cross-sectional area of a flow path in the inner pipe 17from the cross-sectional area of a flow path in the extension pipe 14,preferably satisfies the relation represented by the following formula(19) with the cross-sectional area S₁ (mm²) of a flow path in the innerpipe 17.

ΔS≦S ₁  (19)

Here, the cross-sectional area of a flow path in the extension pipe 14or in the inner pipe 17, means a cross-sectional area in a planeperpendicular to the longitudinal direction of the flow path in theextension pipe 14 or the inner pipe 17. When ΔS and S₁ satisfy therelation represented by the formula (19), the width of the gap portionbetween the inner wall of the extension pipe 14 and the outer wall ofthe inner pipe 17 does not become too large with respect to thethickness of the boundary laminar flow, and accordingly, the amount ofmain flow separated to move into the gap portion does not increase.Accordingly, the yield of production of glass does not decrease.

Further, it is preferred that 0.50×S₁≦ΔS is satisfied.

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is preferred that there is an overlap portion. If theoverlap portion is present, the effect of separating the boundarylaminar flow from the main flow increases, such being preferred.

The length L (mm) of the overlap portion and the outer diameter D₂ (mm)of the inner pipe 17 preferably satisfy the relation represented by thefollowing formula (20).

L≧0.5×D ₂  (20)

In the vacuum degassing apparatus 1′ having the structure shown in FIG.7, in order to adjust the height of the level of molten glass G in thevacuum degassing vessel 3, it is necessary to move up and down thevacuum degassing vessel 3 by maximum about 600 mm. At this time, theextension pipe 14 moves up and down in accordance with the movement ofthe vacuum degassing vessel 3. Accordingly, the length L of the overlapportion changes in accordance with the movement of the vacuum degassingvessel 3, and L is minimized when the vacuum degassing vessel 3 moves upto the maximum position.

In all states including the state that L is minimized, L and D₂preferably satisfy the relation represented by the above formula (20).Here, as described above, L may be 0 (that is, the extension pipe 14 andthe inner pipe 17 do not overlap with each other).

Further, since the upper end (upstream end) of the inner pipe may moveinto the extension pipe (downfalling pipe) too much, L preferablysatisfies the following formula (21).

L≦5×D ₂  (21)

When L and D₂ satisfy the relation represented by the above formula (20)in all states including a state that L is minimized, regardless of themovement of the vacuum degassing vessel 3, the length L of the gapportion between the inner wall of the extension pipe 14 and the outerwall of the inner pipe 17, is sufficient for physically separating theboundary laminar flow from the main flow. Further, even if the vacuumdegassing vessel 3 is moved up and down in the maximum range, theleading edge of the extension pipe 14 is always immersed in molten glassG in the downstream side pit 15 (pit main body 16).

D₂ changes depending on the size of the vacuum degassing apparatus,particularly on the flow rate (t/day) of molten glass passing throughthe vacuum degassing apparatus, but it is usually from 51 to 912 mm,more preferably from 102 to 708 mm. L is preferably at least 30 mm andat most 1,000 mm, more preferably at least 50 mm and at most 700 mm. IfL exceeds 1,000 mm, increase of L does not contribute to separation ofthe boundary laminar flow from the main flow any more, and the length ofthe overlap portion becomes extremely long, which increases the cost.

Here, the length of the extension pipe 14 itself is usually from 200 to3,000 mm, more preferably from 400 to 1,500 mm. Since the inner pipe 17extends in the downstream direction in the Figure, the length is notparticularly limited. However, the length of the inner pipe 17 ispreferably from 50 mm to 600 mm, more preferably from 100 mm to 500 mm.

In the third embodiment of the vacuum degassing apparatus of the presentinvention, it is sufficient that the downstream side pit has a doublepipe structure comprising a pit main body being an outer pipe and aninner pipe located inside the pit main body and extending in thedownstream direction, and the double pipe structure is not limited toone wherein the extension pipe 14 overlaps with the inner pipe 17 asshown in FIG. 8.

FIG. 10 is a partial enlarged view showing a downstream side pit and itsvicinity in another example of the third embodiment of the vacuumdegassing apparatus of the present invention. This example is the sameas the example shown in FIG. 8 except that the relation between theextension pipe and the inner pipe is different.

In the example shown in FIG. 10, the extension pipe 14′ does not overlapwith the inner pipe 17′, and the lower end (downstream end) of theextension pipe 14′ is isolated from the upper end (upstream end) of theinner pipe 17′. Since they do not overlap, it is not only possible tosimplify the platinum conduit itself but also to simplify the design ofthe equipment.

Here, no-overlap may be recognized as a simple design change, but it isnot true. It should be noted that there is a difficulty unique to anapparatus for molten glass.

A piece of glass production equipment is continuously operated for anextremely long time (about from 2 to 15 years) once it is assembled andflow of molten glass is started. Accordingly, if any mistake occurs, itis extremely difficult to repair, and a total rebuilding becomesnecessary. Further, since molten glass has an extremely high temperatureof at least 1,200° C., it is extremely difficult to directly observe theflow. Accordingly, an apparatus for molten glass is preferably designedto be an apparatus not having any problem later, and at the same time,to be an extremely simple apparatus in some cases.

This invention of no-overlap contributes extremely significantly in thepoint that the object of the present invention can be achieved withoutoverlap.

When molten glass flow containing the boundary laminar flow reaches thelower end (downstream end) of the extension pipe 14′ shown in FIG. 10, aboundary laminar flow containing many bubbles moves along the directionof an arrow B at the lower end (downstream end) of the extension pipe14′. Namely, from the lower end (downstream end) of the extension pipe14′, it spreads to the outside and moves in a gap between the inner wallof the pit main body 16′ and the outer wall of the inner pipe 17′.Meanwhile, a main flow moves along the direction of the arrow A, andmoves into the inner pipe 17′. As a result, the boundary laminar flow isphysically separated from the main flow.

In the example shown in FIG. 10, the reason why the boundary laminarflow moves along the direction of the arrow B, is described as follows.

In a portion between the lower end (downstream end) of the extensionpipe 14′ and the upper end (upstream end) of the inner pipe 17′, apressure difference is formed between the central portion of the pitmain body 16′, in which the main flow flows, and the outer periphery (inthe vicinity of the inner wall) of the pit main body 16′, whereby thepressure along the outer periphery (in the vicinity of the inner wall)of the pit main body 16′ is lower than the pressure in the vicinity ofthe center of the pit main body 16′. By this pressure difference, theboundary laminar flow moves along the direction of the arrow B.

In the example shown in FIG. 10, the main flow moves along the directionof the arrow A in the Figure into the inner pipe 17′, and moves in thedownstream direction. Meanwhile, the boundary laminar flow moves alongthe direction of the arrow B in the Figure into the gap portion betweenthe inner wall of the pit main body 16′ and the outer wall of the innerpipe 17′, and moves through the opening provided in a side portion (sidewall) of the pit main body 16′ into the returning pipe 11. Thus, theboundary laminar flow is physically separated from the main flow, andonly the main flow from which bubbles are sufficiently removed by vacuumdegassing is supplied to a forming apparatus. Meanwhile, the boundarylaminar flow containing many bubbles moves in the returning pipe 11, tobe sent to the upstream side pit 9. The boundary laminar flow that hasreached the upstream side pit 9, moves through the uprising pipe 4 (morespecifically, the extension pipe 7 and the uprising pipe 4) along withmolten glass newly supplied from a melting vessel 100, to be sent to thevacuum degassing vessel 3.

In the example shown in FIG. 10, the distance d (mm) between the lowerend (downstream end) of the extension pipe 14′ and the upper end(upstream end) of the inner pipe 17′, preferably satisfies the relationrepresented by the following formula (22) with the outer diameter D₂(mm) of the inner pipe 17′.

0≦d≦5×D ₂  (22)

When d and D₂ satisfy the above formula (22), the distance between thelower end (downstream end) of the extension pipe 14′ and the upper end(upstream end) of the inner pipe 17′, is sufficient for physicallyseparating the boundary laminar flow from the main flow. Morespecifically, when d and D₂ satisfy the above formula (22), the boundarylaminar flow moves along the direction of the arrow B, while the mainflow moves along the direction of the arrow A. Accordingly, there is norisk that a part of the main flow moves along the direction of the arrowB, and that a part of the boundary laminar flow moving along thedirection of the arrow B merges again with the main flow.

d and D₂ preferably satisfy the following formula (23), more preferablysatisfy the following formula (24).

0.5×D ₂ ≦d≦4×D ₂  (23)

0.5×D ₂ ≦d≦2×D ₂  (24)

D₂ is defined in the same manner as one in the example shown in FIG. 8,and it is usually from 51 to 912 mm, more preferably from 102 to 708 mm.d is preferably at least 30 mm and at most 1,000 mm, more preferably atleast 50 mm and at most 700 mm.

Here, the dimensions of the extension pipe 14′ and the inner pipe 17′are similar to those described in the example shown in FIG. 8.

FIG. 11 is a partial enlarged view showing a downstream side pit and itsvicinity in another example of the third embodiment of the vacuumdegassing apparatus of the present invention. In the example shown inFIG. 11, the shape of the upper end (upstream end) of the inner pipe 17″is different from that of the inner pipe 17′ of FIG. 12. Namely, in theexample shown in FIG. 11, a diameter-expanding portion 18 is provided atthe upper end (upstream end) of the inner pipe 17″. In the example shownin FIG. 11, since the diameter-expanding portion 18 is provided at theupper end (upstream end) of the inner pipe 17″, it is possible toincrease the flow rate of the main flow while design change for theequipment is minimized.

Here, the diameter-expanding portion 18 is not limited to one shown inFIG. 11 wherein the diameter decreases rapidly, but it may be onewherein the diameter decreases slopewisely or stepwisely.

In the example shown in FIG. 10 or FIG. 11, the inner diameter D₁ (mm)of the extension pipe 14′ and the outer diameter D₂ (mm) of the innerpipe 17′ or 17″, preferably satisfy the relation represented by thefollowing formula (25).

0.98×D ₂ ≦D ₁≦2.5×D ₂  (25)

Here, as shown in FIG. 11, when a diameter-expanding portion 18 isprovided at the upper end (upstream end) of the inner pipe 17″, theouter diameter D₂ of the inner pipe 17″ means the outer diameter of thediameter-expanding portion 18.

When the inner diameter D₁ of the extension pipe 14′ and the outerdiameter D₂ of the inner pipe 17′ or 17″ satisfy the relationrepresented by the above formula (25), the difference between the innerdiameter of the extension pipe 14′ and the outer diameter of the innerpipe 17′ or 17″, is not significantly large, such being suitable forphysically separating the boundary laminar flow from the main flow. Whenthe difference between the inner diameter of the extension pipe 14′ andthe outer diameter of the outer pipe 17′ or 17″, is significantly large,it may not be possible to sufficiently separate the boundary laminarflow from the main flow (when the outer diameter of the inner pipe 17′or 17″ is large). Further, the amount of the main flow separated intothe boundary laminar flow side increases to reduce the yield ofproduction of glass, such being not preferred (when the inner diameterof the extension pipe 14′ is large).

Next, a fourth embodiment of the vacuum degassing apparatus of thepresent invention will be described.

FIG. 12 is a cross-sectional view showing the fourth embodiment of thevacuum degassing apparatus of the present invention. The vacuumdegassing apparatus 1″ shown in FIG. 12 is the same as the vacuumdegassing apparatus 1 shown in FIG. 1 except for the followingdifferences.

-   -   The structure is not such that the lower ends of the extension        pipes connected to the upstream pipe and the downstream pipe,        respectively, are immersed in molten glass in the upstream side        pit and the downstream side pit, respectively, but the structure        is such that the upstream pipe 4′ and the downstream pipe 5′        communicate with the upstream side pit 19 and the downstream        side pit 20, respectively, so as to be filled with molten glass        (difference 1).    -   There is no double pipe structure functioning as a separating        mechanism that is present in the first to third embodiments of        the vacuum degassing apparatus of the present invention        (difference 2).

The vacuum degassing apparatus 1″ shown in FIG. 12 has a structure thatthe uprising pipe 4′ and the downfalling pipe 5′ are connected with theupstream side pit 19 and the downstream side pit 20, respectively, so asto be filled with molten glass, whereby the structure has such meritsthat the structure is rigid and the cost for building the apparatus canbe saved.

However, since the vacuum degassing apparatus 1″ shown in FIG. 12 has astructure that the uprising pipe 4′ and the downfalling pipe 5′ areconnected with the upstream side pit 19 and the downstream side pit 20,respectively, so as to be filled with molten glass, it is not possibleto maintain the level of the molten glass G in the vacuum degassingvessel 3 constant by moving up and down the vacuum degassing vessel 3when the degree of vacuum in the vacuum degassing vessel 3 iscompensated. Accordingly, when the level of molten glass G in the vacuumdegassing vessel 3 changes, the change affects the effect of vacuumdegassing. Particularly, when the level of molten glass G in the vacuumdegassing vessel 3 rises, the vacuum degassing effect is decreased tocause a problem that a boundary laminar flow containing many bubblesincreases.

However, in the vacuum degassing apparatus 1″ shown in FIG. 12, aboundary laminar flow containing many bubbles is separated by aseparating mechanism from a main flow of molten glass G moving from thedownfalling pipe 5′ to the downstream side pit 20, and the boundarylaminar flow is returned through a returning pipe 11 to the vacuumdegassing vessel 3. Accordingly, it is possible to suppress theinfluence of decrease of vacuum degassing effect.

However, in the fourth embodiment of the vacuum degassing apparatus ofthe present invention, the difference 1 is not an essentialconstruction, but in the same manner as the first to third embodimentsof the vacuum degassing apparatus of the present invention, theconstruction may be such that the lower ends of the extension pipesconnected to the uprising pipe and the downfalling pipe, respectively,are immersed in molten glass in the upstream side pit and the downstreamside pit, respectively.

In the fourth embodiment of the vacuum degassing apparatus of thepresent invention, the opening of the returning pipe 11 opening in thedownstream side pit 20 so as to satisfy the following conditions (1) and(2), functions as a separating mechanism for separating a boundarylaminar flow containing many bubbles from a main flow of molten glass Gmoving from the downfalling pipe 5′ to the downstream side pit 20.

(1) The opening crosses a part of an imaginary region obtained byimaginarily extending the downfalling pipe 5′ in the downstreamdirection.

(2) The opening does not cross an imaginary line obtained by imaginarilyextending the central axis of the downfalling pipe 5′ in the downstreamside.

The above conditions (1) and (2) will be described with reference toFIG. 13. FIG. 13 is a partial enlarged view showing a downstream sidepit and its vicinity in a vacuum degassing apparatus 1″ shown in FIG.12.

In FIG. 13, the returning pipe 11 extends from a left side of adownstream side pit 20 in the Figure, that is, from the vicinity of thelower end of a side portion (side wall) in the upstream side in thehorizontal direction, towards the upstream side in the horizontaldirection. An opening 22 (represented by a broken line) of the returningpipe 11 is provided in the vicinity of a lower end of a side portion(side wall) on the upstream side in the horizontal direction of thedownstream side pit 20, and crosses a part of an imaginary region 23(represented by the hatched portion) obtained by imaginarily extending adownfalling pipe 5′ in the downstream direction. Here, in thisspecification, the imaginary region obtained by extending thedownfalling pipe in the downstream direction, means, as shown in FIG.13, not a region obtained by extending the outer diameter of thedownfalling pipe 5′ in the downstream direction, but a region obtainedby extending the inner diameter of the downfalling pipe 5′ in thedownstream direction. By such a construction, a boundary laminar flowflowing along a wall face of the downfalling pipe 5′ on a left side ofthe Figure, that is, along a wall face of the downfalling pipe 5′ on theupstream side in the flow direction of molten glass G in the horizontaldirection, is separated from the main flow, to move through the opening22 to move into the returning pipe 11.

In FIG. 13, the opening 22 does not cross an imaginary line 24(represented by a broken line) obtained by imaginarily extending thecentral axis of the downfalling pipe 5′ in the downstream direction. Bythis construction, it is possible to efficiently introduce the boundarylaminar flow portion from the main flow into the returning pipe 11.

Accordingly, in the fourth embodiment of the vacuum degassing apparatusof the present invention, in the boundary laminar flow flowing along thewall face of the downfalling pipe 5′, a boundary laminar flow flowingalong a wall face of the downfalling pipe 5′ in the upstream side in thehorizontal direction, that is on the left side in the Figure, isseparated.

As described above, the reasons why bubbles in molten glass increase inspite of carrying out vacuum degassing, are generation of bubbles on theinterface between molten glass and a wall face of a conduit for moltenglass, and lowering of vacuum degassing effect due to rise of level ofmolten glass in a vacuum degassing vessel.

For glasses such as glasses for buildings or automobiles for which thebubble quality requirement is not very strict, it is sufficient that aboundary laminar flow containing bubbles mainly caused by the secondreason is separated and returned to a vacuum degassing vessel to besubjected to vacuum degassing again.

As described above, the boundary laminar flow produced by lowering ofvacuum degassing effect due to rise of molten glass level in a vacuumdegassing vessel, flows along a wall face of a downfalling pipe 5 on theupstream side in the horizontal direction of the pipe, that is on theleft side of the pipe in the Figure, and accordingly, it is possible toseparate such a boundary laminar flow from a main flow by using thefourth embodiment of the vacuum degassing apparatus of the presentinvention. Further, in a boundary laminar flow produced by generation ofbubbles on the interface between molten glass and a wall face of aconduit for molten glass, a portion flowing along a wall face of thedownfalling pipe 5′ on the upstream side in the horizontal direction ofthe pipe, that is on the left side of the pipe in the Figure, can beseparated by using the fourth embodiment of the vacuum degassingapparatus of the present invention.

On the other hand, for such a glass to be employed for flat paneldisplay panels for which the bubble quality required for the glass isextremely strict, it is preferred to employ the first to thirdembodiments of the vacuum degassing apparatus of the present inventionfor separating a boundary laminar flow from a main flow by a double pipestructure, to separate a boundary laminar flow caused by generation ofbubbles on the interface between molten glass and a wall face of aconduit for molten glass as well as a boundary laminar flow caused bylowering of vacuum degassing effect due to rise of molten glass level ina vacuum degassing vessel.

Here, when any of the first to third embodiments of the vacuum degassingapparatus of the present invention is employed, a boundary laminar flowforming a surface layer of a flow of molten glass G in the vacuumdegassing vessel 3 can also be properly separated. In some vacuumdegassing conditions, non-broken bubbles are present on a surface ofmolten glass in the vacuum degassing vessel, which form a surface layerof the flow of molten glass G moving in the vacuum degassing vessel.

When the flow of molten glass G moves to the downfalling pipe, thesurface layer of the flow of molten glass G containing non-brokenbubbles tends to U-turn along a wall face of a downstream end of thevacuum degassing vessel and move into the downfalling pipe. As a result,rather than a boundary laminar flow flowing along a wall face of thedownfalling pipe on the upstream side in the horizontal direction of thepipe, a boundary laminar flow flowing along a wall face of thedownfalling pipe on the downstream side in the horizontal molten glassflow direction (hereinafter referred to as “downstream side inhorizontal direction”) of the pipe, tends to contain more bubbles. Byemploying the first to third embodiments of the vacuum degassingapparatus of the present invention which separate a boundary laminarflow from a main flow by a double-pipe structure, such a boundarylaminar flow can also be suitably separated.

Returning to the fourth embodiment of the vacuum degassing apparatus ofthe present invention, in order to provide an opening 22 so as tosatisfy the above conditions (1) and (2), as evident from FIG. 13, it issufficient that an end of the returning pipe 11, more specifically anend of the opening 22 of the returning pipe 11, is located in theimaginary region 23 and on the upstream side in the horizontal directionfrom the imaginary line 24. In other words, it is sufficient that theopening 22 is provided so that the minimum distance d_(min) (mm) betweenthe returning pipe 11 and the imaginary line 24 satisfies the relationrepresented by the following formula (26) with the radius D_(down) (mm)of the downfalling pipe 5′.

0<d _(min) <D _(down)  (26)

In FIG. 13, an opening 22 is obliquely provided in the vicinity of alower end of a side portion (side wall) of a downstream side pit 20 onthe upstream side in the horizontal direction, but the openingsatisfying the above conditions (1) and (2) may be provided in a bottomof the downstream side pit 20. For example, when the opening is providedin the vicinity of a left end in the bottom of the downstream side pit20, the above conditions (1) and (2) are satisfied. In this case, thereturning pipe extends downwardly from the opening in the Figure. Alsoby such a construction, it is possible to separate a boundary laminarflow flowing along a wall face of the downfalling pipe 5′ on theupstream side in the horizontal direction of the pipe, that is on theleft side of the pipe in the Figure, from a main flow. However, in sucha construction, since it is necessary to bend the returning pipe in themiddle to extend the returning pipe towards the upstream side in thehorizontal direction, the flow resistance of molten glass in thereturning pipe may increase. Further, in a case of a structure whereinan opening of the returning pipe is provided in a bottom of thedownstream side pit, it is not possible to increase the area of theopening and the flow resistance of molten glass in the returning pipe 11becomes high as compared with a structure wherein an opening 22 isobliquely provided in the vicinity of a lower end of a side portion(side wall) of the downstream side pit 20 on the upstream side in thehorizontal direction of the pit as shown in FIG. 13.

Accordingly, as shown in FIG. 13, it is preferred to obliquely providethe opening 22 in the vicinity of a lower end of a side portion (sidewall) of the downstream side pit 20 on the upstream side in thehorizontal direction of the pit as shown in FIG. 13. Here, an angle α(degrees) between the opening 22 and the imaginary line 24 preferablysatisfies the following formula (27).

10≦α≦80  (27)

Here, when the angle α satisfies the above formula (27), theconstruction is excellent in separation of a boundary laminar flow, andthe area of the opening 22 is proper. Further, since the bending angleof the flow path of molten glass entering from the opening 22 into thereturning pipe 11 becomes gentle, the flow resistance of molten glass inthe returning pipe 11 does not increase. Further, from the viewpoint ofseparation of boundary laminar flow and the viewpoint of equipment, theangle α is more preferably at least 20 degrees and at most 70 degrees.

As shown in FIG. 13, when the opening 22 of the returning pipe 11extending in the upstream side in the horizontal direction, is obliquelyprovided in the vicinity of a lower end of a side portion (side wall) ofthe downstream side pit 20 on the upstream side in the horizontaldirection, in order to suppress slack of molten glass in the vicinity ofthe opening 22, it is preferred to make the height of the bottom face ofthe downstream side pit 20 different from the height of the bottom faceof the returning pipe 11 so as to form a step between them. In FIG. 13,the height of the bottom face of the returning pipe 11 is lower than theheight of the bottom face of the upstream side pit 20.

FIG. 14 is a partial enlarged view showing a downstream side pit and itsvicinity of another example of the fourth embodiment of the vacuumdegassing apparatus of the present invention. In FIG. 14, the height ofthe bottom face of the downstream side pit 20 is lower than the heightof the bottom face of the returning pipe 11 contrarily to that of FIG.13.

As shown in FIGS. 13 and 14, in a case of providing a step between thebottom face of the upstream side pit 20 and the bottom face of thereturning pipe 11 in order to suppress slack of molten glass in thevicinity of the opening 22, they are preferably connected by a slopestructure having an angle of from 5 to 60°. Here, the phrase “a slopestructure having an angle of from 5 to 60° as an approximate shape”mainly means a slope structure of a slope shape having an angle of from5 to 60°, but it is not limited thereto, and it includes a stepstructure which can be approximated by a slope structure having an angleof from 5 to 60°. When the angle of the slope structure is within theabove range, it is possible to effectively suppress slack of moltenglass in the vicinity of the opening 22. Further, if the angle of theslope structure is too small, since the length of the slope structurebecomes long, the cross-sectional area of the downstream side pit 20 orthe returning pipe 11 becomes small, or the cross-sectional area of thedownstream side pit 20 or the returning pipe 11 changes in the middle,such being not preferred.

The slope structure connecting the bottom face of the downstream sidepit 20 and the bottom face of the returning pipe 11 preferably has anangle of from 10 to 60 degrees, more preferably from 30 to 60 degrees.

When a step is provided between the bottom face of the downstream sidepit 20 and the bottom face of the returning pipe 11, so long as they canbe connected by a slope structure having an angle of from 5 to 60°, theheight of the step is not particularly limited. The height of the stepis preferably determined so that the area of the opening 22 becomessubstantially the same as the cross-sectional area of the returning pipe11.

Next, the vacuum degassing method of the present invention will bedescribed.

The vacuum degassing method of the present invention is a method forvacuum-degassing molten glass by making the molten glass passing througha vacuum degassing vessel inside of which is maintained in a vacuumstate, which is characterized in that a part of molten glass flowing outfrom the vacuum degassing vessel is separated and the separated moltenglass is returned again to the vacuum degassing vessel. In other words,in the vacuum degassing method of the present invention, whenvacuum-degassing of molten glass is carried out by using the vacuumdegassing apparatus, a part of molten glass flowing out from a vacuumdegassing vessel, specifically a boundary laminar flow containing manybubbles, is separated from molten glass flowing out from the vacuumdegassing vessel, and the separated boundary laminar flow is returnedagain to the vacuum degassing vessel, to carry out vacuum-degassingagain.

Accordingly, the vacuum degassing method of the present invention can besuitably carried out by employing the first to fourth embodiments of thevacuum degassing apparatus of the present invention.

In the vacuum degassing method for molten glass of the presentinvention, the molten glass is preferably continuously supplied anddischarged from the vacuum degassing vessel.

Here, the flow rate of molten glass is preferably from 1 to 1,000ton/day from the viewpoint of productivity.

The ratio of molten glass separated and returned to a vacuum degassingvessel based on the molten glass flown out from the vacuum degassingvessel, depends on the ratio of boundary laminar flow contained in themolten glass flown out from the vacuum degassing vessel, and it ispreferably at most 20% of the molten glass flown out from the vacuumdegassing vessel in order to prevent lowering of yield of production ofglass. The ratio of molten glass separated and returned to the vacuumdegassing vessel based on the molten glass flown out from the vacuumdegassing vessel, is more preferably from 0.1 to 10%, further preferablyfrom 1 to 5%.

The ratio of molten glass separated and returned to the vacuum degassingvessel based on molten glass flown out from the vacuum degassing vessel,can be changed while the vacuum degassing is carried out. For example,at a start of vacuum degassing, since the amount of bubbles contained inmolten glass is large, the ratio of molten glass separated and returnedto the vacuum degassing vessel is set to be high, and later, when thestate of vacuum degassing is stabilized and bubbles are reduced, theratio of molten glass separated and returned to the vacuum degassingvessel may be lowered. The ratio of molten glass separated and returnedto the vacuum degassing vessel can be adjusted by changing the flow rateof molten glass in the returning pipe 11 by a pumping means 12.

Further, the separated molten glass is preferably heated and stirred inthe returning pipe 11 before it is returned to the vacuum degassingvessel.

In order to prevent formation of a temperature difference between moltenglass supplied from a melting vessel and molten glass in the vacuumdegassing vessel, the vacuum degassing vessel is preferably heated sothat the inside temperature is from 1,100 to 1,500° C., particularlypreferably from 1,150 to 1,450° C. When the temperature of molten glassreturned to the vacuum degassing vessel becomes lower than thetemperature of molten glass continuously supplied from the meltingvessel, it is possible to raise the temperature of molten glass in thereturning pipe 11 by a heating means.

When the vacuum degassing method is carried out, inside of the vacuumdegassing vessel disposed in a vacuum housing is maintained to be in apredetermined vacuum state by evacuating air in the vacuum housing tothe outside by e.g. a vacuum pump. Here, inside of the vacuum degassingvessel is preferably evacuated to 51 to 613 hPa (38 to 460 mmHg), morepreferably, inside of the vacuum degassing vessel is evacuated to 80 to338 hPa (60 to 253 mmHg).

A glass applicable to the vacuum degassing method of the presentinvention is not limited in the composition so long as the glass isproduced by a heat-melting method. Accordingly, it may be a soda limesilica type glass such as a soda lime glass, or an alkali glass such asan alkali borosilicate glass.

INDUSTRIAL APPLICABILITY

The present invention is applicable to production of various types ofglasses required to satisfy strict bubble quality.

The entire disclosure of Japanese Patent Application No. 2008-046247filed on Feb. 27, 2008 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

1. A vacuum degassing method for molten glass, comprising: passingmolten glass through a vacuum degassing vessel inside of which ismaintained in a vacuum state; separating a part of the molten glassflown out from the vacuum degassing vessel; and returning separatedmolten glass to the vacuum degassing vessel.
 2. The vacuum degassingmethod for molten glass according to claim 1, wherein an amount of theseparated molten glass is at least 0.1% and at most 10% of an amount ofthe molten glass passing through the vacuum degassing vessel.
 3. Thevacuum degassing method for molten glass according to claim 1, whereinan amount of the separated molten glass is at least 1% and at most 5% ofan amount of the molten glass passing through the vacuum degassingvessel.
 4. The vacuum degassing method for molten glass according toclaim 1, wherein the passing comprises changing a ratio of an amount ofthe separated molten glass to an amount of the molten glass passingthrough the vacuum degassing vessel while the molten glass is passedthrough the vacuum degassing vessel.
 5. The vacuum degassing method formolten glass according to claim 1, further comprising heating theseparated molten glass before the returning of the separated moltenglass to the vacuum degassing vessel.
 6. The vacuum degassing method formolten glass according to claim 1, further comprising stirring theseparated molten glass before the returning of the separated moltenglass to the vacuum degassing vessel.
 7. A method of manufacturing aglass product, comprising: melting a glass material in a melting furnaceto obtain molten glass; passing the molten glass through a vacuumdegassing vessel inside of which is maintained in a vacuum state;separating a part of the molten glass flown out from the vacuumdegassing vessel; returning separated molten glass to the vacuumdegassing vessel; and supplying a main part of the molten glass flownout from the vacuum degassing vessel to a forming apparatus configuredto shape the molten glass.
 8. The method according to claim 7, whereinan amount of the separated molten glass is at least 0.1% and at most 10%of an amount of the molten glass passing through the vacuum degassingvessel.
 9. The method according to claim 7, wherein an amount of theseparated molten glass is at least 1% and at most 5% of an amount of themolten glass passing through the vacuum degassing vessel.
 10. The methodaccording to claim 7, wherein the passing comprises changing a ratio ofan amount of the separated molten glass to an amount of the molten glasspassing through the vacuum degassing vessel while the molten glass ispassed through the vacuum degassing vessel.
 11. The method according toclaim 7, further comprising heating the separated molten glass beforethe returning of the separated molten glass to the vacuum degassingvessel.
 12. The method according to claim 7, further comprising stirringthe separated molten glass before the returning of the separated moltenglass to the vacuum degassing vessel.